Man, you sure get a lot of bang for your buck with this product! By current RPG standards, this book is extremely affordable (the original hardcover version was over $50), and it is extremely well designed in terms of its layout and graphics contents.
Also, it comes with a short prose story as an introduction and a short illustrated tale as well.
Yes, it does present a lot of mandatory game mechanics materials (using the GURPS system) and the usual sections about generating characters and how to use magic, plus a pretty detailed sample adventure. But even a non-gamer might want to pick this up. It has a timeline of Hellboy/BPRD events, great background information on the lead characters (Hellboy, Abe Sapien, Liz Sherman, Kate Corrigan, Roger the Golem, Johann the psychic, plus a lot of supporting folks), and plenty of stuff on the villains.
One of the distinctive features of Hellboy's world is that the BPRD isn't just hunting monsters and demons from folklore and fairy tales. They're also regularly tussling with Nazis. Yeah, those crazy Nazis--a lot of your more eviller ones survived WWII by going into suspended animation, and now they've returned as crazed sado-surgeons creating robot zombies, she-wolves of the SS in search of their vampire lovers, and worse. Probably the most memorable of them is Professor Doctor Herman von Klempt, of whom this book succinctly says "He always had a plan for world conquest, usually involving cybernetically enhanced apes and robot tanks. He ws everything an insane Nazi mad scientist reduced to a head in a tank should be."
You really just can't improve on that prose.
There's also a pretty nifty bibliography that covered the Hellboy material through 2002. A nice added bonus if you're trying to get an idea of what other Hellboy books you'd like to explore.