Should manuals be GPL'd too?

The FSF published a plea recently for the manuals documenting free-software to be placed under a GPL-like license, so that they stay up to date with the development of the programs they are supposed to document. This is a good point, but may contradict my hope that one can work full-time on free-software by living off the royaltees from its documentation. On the other hand, there are books freely available in source format that are also published. I know I buy Linux on a CD for ease of use, and a spiral bound copy of a good book is invaluable, so people would buy it. In fact I'm more likely to buy a book I have read and liked, than an unknown. So maybe it's possible to sell your cake and eat it. What do you think?