Something people may not realize about Japan is that they tend to not care for books which are too thick. This isn't always true of course, and it's easy to find 1000 page strategy guides for the latest Pokemon games if that's your thing, but in general, when a book is too long, they split it in half. Most of the Harry Potter series, for example, are split in half and sold as two volumes each. In addition to doubling the profits of booksellers, I suppose it has the benefit of making the books easier to hold.
Still, it's a little strange that this guide to Phantasy Star III is split into two volumes, since each one is only around 120 pages. Perhaps they weren't quite finished but wanted to get the guide to market while the game was new, so they released what they had as volume 1 and released volume 2 month later once they'd finished? Who knows.
https://archive.org/details/phantasystariiiattackmanualbookvol.1
https://archive.org/details/phantasystariiiattackmanualbookvol.2