This post, however, is different. I am about to actively try to dissuade you from writing a few different kinds of books because they have historically sold poorly, will continue to sell poorly, and if your book falls into one of these categories it is going to sell poorly, regardless of how awesome you think it is. Without further ado and in no particular order:
#1: Poetry. Full disclosure: I am, among many things, a poet. When I'm not at work helping to sell books or at home writing this blog, I'm writing poetry. I also write fiction, but (in my opinion) it's not very good—at least, not as good as my poetry—so I write poems more often than I write stories.
Something I know and that you need to know: poetry does not sell. Period. If you aren't Homer or Billy Collins, you haven't got a shot at decent sales, much less a deal with a major trade publisher. Earning your MFA at Iowa or winning a Pulitzer helps, but by no means guarantees you anything. I write poetry because I like doing it and I want to be recognized for my ability (i.e. through publication, often unpaid, in literary journals). I do not write it in the hopes of ever making enough money to pay my water bill, let alone to serve as a primary source of income.
#2: Short story collections. Same as poetry: people simply don't buy them. Sure, there are exceptions like Flannery O'Connor, Ernest Hemingway, and Kurt Vonnegut, but these writers are one in a million (or fewer). Again, earning your MFA at Iowa or Columbia helps, but not really. The vast, vast, vast majority of MFA candidates graduate without a book deal anywhere in sight. If you're working on a short story collection, cut it out. Write a novel instead.
#3: Christian fiction. I may get flak for this, but I'm being honest: it doesn't sell. While there are some notable exceptions, the vast majority of Christian novels simply don't move—largely because they cater to a niche audience of unusually devout Christians. (Back in late April, PW reported that next year's Christian Book Expo has been canceled, largely due to lack of attendance at the last one.)
#4: Children's/family cook books. I don't even know why people write these, but they do. Be warned: nobody can sell these, not even Rachael Ray. Remember Yum-O? Disastrous. Check the BookScan numbers, if you're so inclined.
#5: Any book catering to a vanishingly small niche audience. If you are writing a nonfiction book about the history of English doorknobs or a piece of literary fiction that requires the reader be intimately familiar with medieval Breton lays, cut it out. (That is, if you're holding out any hopes of actually selling it.) This includes most academic texts. Maybe you'll have a couple hundred copies published by a university press if you're big news in academic circles, but that's about it.
Now, I realize most of you write mainstream fiction (e.g. non-abstruse literary fiction, chick lit, mysteries, thrillers, romances, science fiction, and so on). Bravo! This sells. I just want to be sure that anyone who has any funny ideas about making a cool million with their interrelated short story collection is swiftly disabused of that notion. Not that I enjoy crushing dreams; it's just that those kinds of thoughts aren't realistic.