It's 1e/2e/fiend folios and some from Dragon, White Dwarf and other sources, and a bunch of originals as well. I think they did a decent job taking the raw abilities and feel of each creature and interpreting it in 5e fashion. It's obvious a lot of thought was put into making the stats useful and faithful to the originals so I find that a good value. Saves me work and time? OK!
Fifth Edition Foes is 260 pages, I don't think they've released a page count for the others.