First the thickness of the wood is variable. Since the two woods do have different tones it is more up to you. The more alder, the more alder the guitar will sound, the more or thicker the maple the more maple. Here is good sight to find out about tones of different woods http://www.warmoth.com/guitar/options/options_bodywoods.cfm
As for book matching book matching is when you cut from one board two pieces of wood that would have been on top of each other, so they have more or less the same grain pattern. Then you set the pieces so that they mirror each other. On a guitar top this means you take two pieces mirror them as explained and then glue them along the edge. This is then glued onto the back piece. This is very common with maple as it has an excellent grain, also bocote is excpetional for this. Also note you can cut a piece of wood as described to bookmatch or buy pre book matched pieces then glue them together. Often you will see pieces sold as bookmatched.
Also anyone may correct me if I am wrong