Sure, conservative books — including “Unfit for Command,” this week’s No. 1 nonfiction title — have lately occupied a hefty percentage of the best-seller list. But at a Manhattan forum this week, right-wing authors said they were feeling marginalized. “Alleging a sort of liberal conspiracy to keep conservative authors from getting their books to the reading public, conservative authors said they had been forced to turn to scrappy, little-known alternative publishers.”