Though these BPRD collections may seem to be stand-alone (they each collect a 6-issue mini-series of the same name), they really must be read in order starting with Volume 3 for the entire Plague of Frogs storyline. I started with Volume 4 because I found it cheap and wanted to give Guy Davis a chance, quickly fell in love, and have now just ordered Volume 6 (after having bought and read through Vol. 3 and 5).
Guy Davis' art really adds a lot to this story. He designs these nightmarish creatures very well, and really nails the scale necessary for these terrors. The colorist, Dave Stewart, cannot be overlooked for his contribution, either; his palette serves Davis' style very comfortably.
Story-wise, I'm hooked. This is an epic story in terms of scale and impact on our heroes, and it's a good sign when I keep finding excuses to spend money on the next trade ASAP after finishing the last. The Black Flame in particular adds some interesting stuff to the overall. First off, I was unaware of the deeper connection shared by Roger and Liz up to this point. Sure, I knew that they had both swapped "life-force" in the pages of Hellboy, but I didn't know that that meant they had some sort of constant psychic link. This link is revealed when Liz senses something is wrong with Roger when he's off hunting frogs.
Also of interest is Roger's sudden gung-ho (and surprisingly effective!) leadership, modeled after team leader Ben Daimio. Why does Roger suddenly adapt his attitude and demeanor, and how does it make him an actual leader? Liz notes that Roger took after Hellboy when he was still around, so now in Hellboy's absence Roger has adapted Daimio's character instead.
Which, of course, again makes me wish Hellboy would make an appearance (or even stay a while!). I miss Hellboy's presence just as much as the actual BPRD team does; he is one of the strongest characters in that universe and he added a totally unique dynamic to the decision-making and outlook of the team. There are no rules that say Hellboy can't be in the BPRD book, so let's have him visit, yeah? If he can guest-star in all sorts of other books (Savage Dragon, Ghost, Goon, etc.) surely he can guest star with his own ex-team. You'd think with such an epic event going on, he'd be drawn to it anyhow, especially since it involves his history and destiny.
Back to The Black Flame. Another interesting aspect of the story is Daimio's reaction to Roger's situation at the end. I'm itching to hear more of his back-story, because you know there's some good stuff to tell. He's not quite the mystery that Lobster Johnson is (since we've seen more of his personality), but still: the scar, his "death," his military history. We don't know anything yet!
I recommend this trade only if you're planning on picking up Vol. 3-7 to get the entire story. It's great stuff, and you gotta get the whole picture.