For those never-read-anything-besides-physical-paper people, the Apache Camel user guide in PDF form may interest you. Be warned for printing it though as its quite large :)
Apache Camel User Guide version 1.6.0
This is really the closest thing we have to a book on Camel. Hoping that will change in the near future!
Another great source for nicely printable Camel documentation is FUSE Source. Check it out here.