The Filipino Martial Arts as Taught by Dan Inosanto. This was my first book into exploring the Filipino martial arts.
It has references on how to hold a stick, how to put power into your strikes, zoning, triangular footwork, blocks, parries, an explanation of the alive hand, flow, ways on how to train alone, etc.
Note that this is an old book and that there are some present reads that are very good as well, but Inosanto's book has ALWAYS been a "go back and read" book for me.
Just know this. A book does not take the place of an instructor. The right instructor can be much more interesting and illustrating than any book can ever be.
For Modern Arnis, if that is what he'll be teaching, the Yellow Book is a good reference for basic material (and is often available at the larger bookstores, though not always in classic yellow) and Mr. Anderson's books are excellent for more detail (I have several of them). There are also many DVDs out there for the art.