This discussion on themed books is rather interesting:
I have to admit, I've noticed this alot too. And actually I quite enjoy those books when I come across them. I recently read Catherine Barnette's Into Such Perfect Spheres these Holes are Pierced, and liked its narrative through-line alot. In fact, I think with what I've done with errata and am attempting with girls reading novels makes me guilty of what he's talking about. I think, in my case, since the first two manuscripts were rather broad and general, I'm just trying to do something a little more focused than I've done before. And hell, after I'm finished with this, I'm planning on doing some sort of novel-in-verse, sustained narrative sort of thing. My instructor for the Sexton/Oliver/Olds class mentioned last semester that alot of poets later on start to move more toward "projects," like Sexton with Transformations and The Death Notebooks.
But to be honest, even my first two manuscripts have over-arching themes, if not something blatant. almanac is predominantly more rural, wistful, coming of age poems, while fever poems is more surreal, more urban, more....I don't know..fevered. Now, with the latest book, I'm working toward a specific thematic overlay, largely because I find myself approaching the idea of the book as the goal, moreso than the individual poem. I used to just write poems and then draw them together into a manuscript. Now I'm writing poems to go into a book--though of course not ONLY for the book, but in large part, most of the new pieces will go into it. I find it somehow focuses me...even if there's not even the chance anyone will publish these manuscripts at all.
Though it does make one wonder why these themed books are winning a number of contests, why they're being released so often by presses. I think editors and judges are looking for something cohesive, and these books, by their nature HAVE to be cohesive. They can't really fail in that regard. Plus, in terms of potential sales, people seem to like books that are ABOUT something, even if it's sort of general. It's easier to pitch a book like this when some editor or potential reader asks you "Hey, what's this book about?"