Here are books that have been suggested before, as well as a bit on each to kick things off:
Secret of NIMH: This was my suggestion, made at the same time as Howl's Moving Castle actually, and for partly the same reason - the gap between the book version and movie version interested me, and made me wonder how others might react to the books only having seen the movie before, and so on. Plus it's just an interesting book. I'm wondering if it's too long though - I remember it seeming like a bit of a long read back when I first read it, but that might have been partly age, and partly due to one section that dragged.
some vague Urban Supernatural: As was pointed out in previous comments, this probably isn't going to happen unless it gets narrowed down into something less vague, and also possibly not even then. But I'll at least leave it on the list to see what others have to say... I vaguely remember the "Greywalker" series was suggested at one point, and I myself would lean toward either the Mercy Thompson novels (well, the first one I guess), or the Seanan McGuire October Daye books. Oh right, and looking back, The Devil You Know was also specifically suggested, due to Mike Carey being awesome. (And also having written one of the other works we've read for this - Lucifer volume one...)
As a side note, there was also the suggestion to read something just for the giggles, to trash it, like Anita Blake. I had also suggested something of the sort, for Twilight, but that didn't seem to go over well? Well, I thought I'd just throw the idea back out there though, since it cropped up again.
The Jungle Book (or Just So Stories): One of my suggestions, which came to mind mostly because of being a) short overall, and b) already broken up handily into stories. Kipling is of course certainly not without his faults, but that might just make for more interesting discussion.
One of the James Herriot books: Again, one of my discussions, mostly because I was going back to read some, and thought "hey, these are all nicely gathered into sections as well, and might work!" I'm just not sure if they'd be suitable, given the... very clinical nature of some of the stories, but hey, we're all mature adults here, right?
Various classics: These were mentioned before, but vanished from the list again at some point. Some of the things bandied about were some of the sci-fi originals, such as Verne or Wells (which might also work with having read League of Extraordinary Gentlemen recently). I think Mark Twain also may have been mentioned? Any specific works people would like to see go into the pot?
Astro City: I suspect this is probably going to be either the book for June, or one of the ones coming up soon anyhow, since it's been toward the top of the polls lately and barely been edged out. And it is probably time for another trade paperback, so I'm going to kind of highlight this one for June. (I haven't read it myself yet, so I can't really say "omg we should totally do this!" or anything. I am interested in reading it though, just have never gotten around to it.)
Queen and Country: Another one that I didn't suggest and haven't read. Although I did enjoy Rucka's other work that I've read, he's a good writer in general, so I'd tentatively be for this, as well.
Bone: As was mentioned in other comments, this one might not be as appropriate, due to the fact that each TP is really only part of one looong story, and that story doesn't really take off until later, so reading just the first TP might be awkward and less fun. I would tend to go along with that, from what Bone I have read, back in the day. I would lean away from this one, but not so much that I'm going to just yank it off the list without seeing what others might have to say. :p
Marvel TP of some flavor: This was mentioned just because we've done various other stuff, but nothing by Marvel! I don't think we got into any specific suggestions though beyond "maybe something X-Men?" or some such. The Dark Phoenix Saga and From the Ashes are both pretty impressive works, although I'm not sure how well they'd read for someone not all that familiar with the X-Men. (Then again... what with the movies and all, I think most people are at least overall familiar with the X-Men?)
Some things that have been mentioned more recently in comments:
Soon I Will Be Invincible by Austin Grossman: This wasn't my suggestion, and I haven't read it, so I can't really say anything yay-ish or nay-ish. :p It seemed to have tentative support, though...
The Hobbit Since a) classic, and b) the movie's coming out soon. I seem to remember it being a bit long though, but again, that might be just due to being younger when I read it. (And come to think of it, not sure if I've actually read it since, or at least more than once since then.)
Sandman volume 2: On the one hand, the series is of course awesome, but I'm not sure if we want to double-dip. (I think we'd also suggested another Austen and another Christie at one point, but they kind of got shot down.) Well, whatever, it's up to everyone! So consider the topic opened about whether we might want to go back to later volumes of some of the things we've already done. And if so, which particular things we might want to read.
Alright guys, so what am I forgetting? What else should be added? Anyhow, please discuss!