In my opinion, "The All-Seeing Eye" is the best of the Hellboy novels I have read thus far.
The strongest points are the plot and the fact that the 3 most famous BPRD characters are involved (Abe, Liz, and of course Hellboy). The plot is a fantastic supernatural mystery, taking cues from various cultures/cults, and the way the team must solve it is suspensful and leaves you hanging for more with each chapter. Hellboy of course is the star, and not much can be said about that. However, Abe and Liz are used perfectly. Instead of just being there as I initially feared, they ALL contribute to solving the mystery, more so the later as her role becomes quite essential.
Of course, you get a bunch of monsters too, what Hellboy story doesn't? But the monsters are secondary to the cult working to bring about the portal to their world, as the monsters are just animals; the people are the ones bringing them here.
One new character that sticks out is Proctor, a reporter for a tabloid newspaper in London. He doesn't contribute much to the story, but his activities help to give the BPRD another challenge: The Press. He's not a good guy, but to see the story unfold from his point of view is rather interesting.
On top of that, the book has a rather bittersweet ending. Can't tell you what, as it would ruin the fun of course.
If you like Hellboy, get this. NOW. I'd even recommend this to people who aren't familiar with the character but like horror in general. Even those not followers of the comics but enjoyed the movies will get a delight out of this.