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.
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.