I have all 3 of the art books, it's a lot of repeated art from the Soejima works, but the P3 and P4 books have a lot concept art, and a bunch of talk about the designs and some interviews with Soejima. If you just want the art, you can't go wrong with just the Soejima works book, because all it is is full, completed pieces.
The P4 book is roughly the same length as Soejima works (a little longer actually, but it's more wordy), but the P3 book is about half the length of them, and nothing from Portable.
The P4 book is roughly the same length as Soejima works (a little longer actually, but it's more wordy), but the P3 book is about half the length of them, and nothing from Portable.