I have said it many times. You can never be too rich, too skinny or have too many monsters.
The Monster Manual was the book for me. The one that got me hooked. The one, sitting in "silent reading" back in 1979 at Washington Elementary School in Jacksonville, IL that I became the über-geek you all know today. How über? I used the freaking umlauts, that's my street cred right there.
Back in '79 I was reading a lot of Greek Myths, I loved reading about all the gods, goddess and monsters. So I saw my friend's Monster Manual and saw all those cool monsters and I knew I had to have a copy. Though getting one in my tiny near-bible-belt town was not easy. Not hard mind you, by the early 1980s the local book store stocked them, but I was not there yet. So I borrowed his and read. And read. And read. I think I had the damn thing memorized long before I ever got my own game going.
Look at it. That is pure awesome still.
Since that time I judge a game book on the "Monster Manual" scale. How close of a feeling do I get from a book or game compared to the scale limit of holding the Monster Manual for the first time? Some games have come close and others have hit the mark as well. C.J. Carella's WitchCraft gave me the same feeling.
Since that time I also like to go to the monster section of any book, or get their monster books. Sure I guess sometimes there are diminishing returns, Monster Manual V for 3.5 anyone? But even then sometimes you get a Fiend Folio (which I liked thankyouverymuch).
This book captured my imagination like no other game book. Even the 1st DMG which is a work of art had to wait till I was older to appreciate it. The Monster Manual grabbed me and took me for a ride.
Of course the real reason to my puberty influenced brain might have been the picture of a naked succubus.
So difficult to know for sure.