This is an interesting video clip-
Part 1 - https://www.youtube.com/watch?v=uhQuJTc5yFY -
Part 2 - https://www.youtube.com/watch?v=1fDij2CWLZY -
that relates to guitar making as one always gets some interesting repairs.
This is one of Willy Nelson's favourite guitars 'Trigger' that was sent in to keep it going as long as possible. This is truly passion.....
Book matching is done when preparing to glue two or more pieces of wood. 
This sometimes works better when there is an even number of pieces. In guitar making, when selecting wood for a top, back and sides for a guitar, they will normally come in two pieces per set. It may be that the wood you select is a block. If it is a single piece, it needs to be thick enough to cut in two. In both cases, they will be joined in what is known as book matched, mirror imaged, or butterflied join. Book matched means that the two pieces paired together form a mirror image. Taking the two pieces and placing them as they were, when it was one piece of wood, the one piece will be turned over, the same as turning the page of a book and this is how the two pieces will be joined the two pieces will mirror each other.
The reason
Regarding the top - firstly, the wood will be similar in terms of the density on both sides of the join,
the grain of the cut will be the same in the middle, normally placing the tighter grain in the centre and the wider grain on the ends giving the wood a very balanced tone as well as giving the newly joined top and back an aesthetically pleasing look. This applies to any wood that is used.
In a previous article, I spoke about Burls in wood. 
A while ago I had someone ask me what is Crotch Mahogany. Where Burls are as a result of growth, of which the cause is often unknown, Crotch, or Flame Mahogany as it is also sometimes referred to, is a result of the way the tree grows. It is where the tree forks and a V is formed, much the same as the crotch of humans. If the tree is cut in the torso, just below the chest and just above the knee (as would be on a person), the wood if then cut or sliced into veneer, would result in the grain forming a type of V or U and the grain would be somewhat more pronounced forming, if you used your imagination, what may represent a flame. Crotch is not only found in the Mahogany species, but in any tree. The size of the tree would matter as the tree would have to be of a fair size for there to be adequate growth to result in the pattern to be of significance.
It is a very distinctive and beautiful feature that is often sought after when making furniture. It would often be used in panelling, door panels, table tops, somewhere where the space or sizes large enough to allow it to be shown off. In solid it would be somewhat of a waste, because you would only see one surface and the repeating visual pattern would be locked in the rest of the piece. So, in this case it would make sense to slice it into veneer, or cut it into thin planks/pieces. It is not often used in guitar making as the size of the instrument would not show off the full beauty, but having said that, if one finds a reasonable specimen and the size and pattern is good, then, a very unique instrument could be made with a good aspect of added value. 
The visual impact is often highlighted when book matched, as in most veneering pieces; the mirror image often improves the effect. I will discuss book matching in a future blog.
It often forms on a tree as a type of nodule or lump and in that lump is an intricate growth system which results in a beautiful, attractive unique pattern when the wood is cut into planks, or sliced into a veneer. Mostly, the burls grow beneath the ground, but when they are above ground level, they are quite noticeable in the form lumpy protrusions on the trunk of the tree. Obtaining the burl wood is expensive because of it scarcity.
Well, no one can say for sure, but it is almost like a type of malignancy or disease that forms causing the tree to create this lump or effect. Sometimes insect infestation or mould infestation can also cause this. In a nutshell, the burl will result when the tree undergoes some kind of stress.
The results of this burl are visually very attractive and very unique and can sometimes give the wood a very wild, busy look. Some common species you may have heard of is, Walnut Burl, Bird's Eye Maple, Elm Burl, Oak Burl, Poplar Burl and there are many others.
This busy, wild grain, makes the wood very dense, which also makes it much harder than the species that it originates from.
This makes it very sought after for making bowls and in the past, it was used for mallets, mauls and chisel handles used for hammering. Today it is used for instruments,inlays in tables, door panels, wooden wall paneling, dash boards in up-market cars, boat building etc. The burl wood is difficult to work with, because of its intricate knurled grain that has no specific direction and when sliced into veneer, it can easily buckle and crack or split. Having said all this, the end result, when it has been used in guitar making, furniture, panelling,or other types of instruments, the result is truly stunning, with a beautiful depth and lustre, as well as being unique, something that only nature can provide.
Are veneers used in guitar making?
The answer is yes. In the lower price range of guitar and even the entry level of the more top well-known brands, veneers are used on the tops, sides and backs. These are known as laminated tops and backs. This is done primarily to cut production costs due to the high volumes produced compared to increasing scarcity of good solids used in instrument making. This makes the instrument look as if it was made in solid and often these instruments, if made well, have a fairly good sound. Some manufacturers also use high pressure laminates (HPL) on their backs and sides. HPL is a laminate that is made under very high heat and pressure, and example of this is Formica, which is covered with a paper foil and then covered with a melamine.
On instruments that are handmade by individual luthiers, they are made using well-chosen tone woods that are solid, the top being the focus of creating an above average tone.
But, many people are not familiar with what a veneer is, so I thought I would give a brief outline of what veneering is.
What is a wood veneer?
Veneer is a thin slice of solid wood that is glued onto a substrate, like chipboard, mdf (medium density fibre), plywood etc. The thickness can vary up to approximately 2mm - 5mm. 
When people who are specialists in selecting veneer, choose logs for slicing that are the prime. The reason for this is that they will get the best optimisation out of the log and hence the quality will be good a minimal defects such as knots, crack, rot, defective grain etc. The word veneer, according to Merriam Webstera's dictionary dates to 1702. However, adding a thin sheet of a superior wood grain to a lesser quality wood is a much older craft. Today, adding a veneer of wood is commonplace on many high-quality solid-wood furnishings as well as less expensive items constructed from particle boards. A simple wood veneer can unify the appearance of a piece or serve the design of the furnishing as a work of art.
How far back does veneering go? The ancient Egyptians were the inventors of veneer wood according to the David R. Webb Company, a modern manufacturer of wood veneers. Though the Egyptians enjoyed the beauty of wood, trees were scarce in the Nile region. By carefully cutting exquisite wood into thin sheets, it could be crafted into delicate boxes, inlaid or applied to the top of less beautiful wood or materials. Ken Melchert of the Harp Gallery notes that fabulous veneer work in ebony” a dense, black wood, was put into King Tut’s tomb.
So, as you can see, veneering is not new, but what has changed, is the substrates and glues used today are more efficient and stable. Also, it is a lot more environmentally friendly, compared to a 1 inch (25mm) thick piece of solid, you get approximately 30 - 40 times the area using veneer, which the standard thickness that is cut today is 0,6mm.
Wood is becoming a scarce commodity. In the furniture and decorating industry, there is definitely a need for veneers. Many of the top manufacturers use it for the consistency and variation that is available it is definitely not inferior, in fact,if done properly, in many instances it can be better.