I just came across an older (back from 2003) post by Jason Kottke who had a horrible thought on including ads in books to drive the prices down. Here is a Russian publishing house (the page is in English) that lets you place ads in their books, mostly on geeky topics. Listen to the pitch: "The book is used for years. Because of it advertisement placed in the book is long-term and highly effective kind of advertisement. We invite you to place your advertising media in our books." And these Ukranian guys (site in Russian) who publish self-help books and Idiot's guides, also accept ads, in color for front and back covers, and in b/w for inside pages. Prices vary between $400 to $1600 for a full page ad.
In the States, STARbooks say they place ads in erotic literature. And here's a wonderful Museum of Comic Book Advertising.