Just thought I'd suggest another one for the library. This one's a Taunton Press (publishers of Fine Woodworking) book titled Understanding Wood: A Craftsman's Guide to Wood Technology by R. Bruce Hoadley.
This is a very serious book, in the category of "everything you wanted to know about wood but were afraid to ask." It includes chapters on wood figure, identification (with lots of microphotographs of end grain), dimensional change, machining, bending, drying, finishes and adhesives. The author does delve into some elementary engineering topics in the chapter on strength, but these aren't extremely hard to understand as there are plenty of drawings and diagrams to supplement the text. Please don't let that one chapter put you off though; doing the math isn't necessary to understanding the rest of the book, and anyone with the ability to build a guitar can muddle through it and glean at least a sense of what's going on. The chapter Water and Wood alone is worth the purchase price.
It's primarily written for those in the construction and fine furniture trades, but I just can't help thinking of this one as an essential book in our hobby. Wood is the raw stuff we build our guitars from after all, and I can't help thinking that understanding it is every bit as important as knowing how to glue a joint or apply a finish.
I picked mine up at the Folk Art Center in Asheville, NC, but I've seen it for sale on Amazon and quite a few woodworking stores...