in reply to (OT) I prefer to do my learning with: dead trees or flying electrons?
I don't know that its essential, but I like my books too much to do without them.
I started trying to learn perl from the online documentation, but I didn't quite get it until I got the books.
I started out with "Learning Perl" and jumped right in with the Camel next. Later, I got the Panther and the Cookbook.
I must agree with the others, though, that the footnotes and the approach and humor of the writing alone make the Camel book worth buying and reading.