[jfoutz]

Really depends on the targeting. For a forum about a book, an affiliate link to by the book or an upcoming sequel makes perfect sense. Recommendations for other books seems pretty fair too. For example, an Ender's game discussion that references Honor Harrington isn't crazy.

Spam about cat litter is just spam.

But it can't be constantly off topic. finding the balance is delicate, and most people do a shitty job.