Probably would have been more suitable for personal message but that's no biggie!
I tried putting a book in his hand but the issue with books in GBA style'd sprites is lack of room. As you probably noticed when you tried yourself, GBA sprites are not very large and a book would roughly be head size so there really is very little space to have character's holding them. Colour limits also cause a bit of a problem due to making it harder for the book to be visible(depending on location).
I did, however, manage to put a book under his left arm(though, it does not look that great).