That is actually the point I was trying to make. If you had a longer book, you could have actually included more information about the extra material used, more than just 'nuggets' and 'tips'.
Most the time when a book uses another source, it will say, if you don't have so and such, just use the wizard class or druid (or whatever). Which is annoying, to say the least. Because even if I do have that book, I probably have to go dig it out of a 7 foot stack of RPG books (literally in my case). And if I don't, well, then likely I'll have to restat the villain/critter myself.
(This was the case in Shadows In Freeport, the villain is a Thaumatauge, the core class from Green Ronin's fiend book. If you don't have it, you have to restat him as a wizard yourself. And SiF was actually pretty good about reprinting material)