Depends on what you're after but you don't have to stop with one. Most of them have been in print so long you can pick up used copies inexpensively. Also, Perl Black Book is underrated (under-known?) and to some degree represents a cross between the Ram and the Camel. It presents each part/function/issue, more or less, as stories which is a great XP technique for approaching problems, even the problem of learning.