While I can’t speak for the reference books, O’Reilly technical books are usually worth the money. Two on this list that I can vouch for are Programming Perl, as I own a hard copy of a previous edition, and Java in a Nutshell which is a rather good book for getting up to speed on the language.

I remember buying the Camel book for about $56 nearly a decade ago. Came for a means of thinking through the documentation that didn’t require alt+tab, stayed for the wonderful wit, anecdotes, and stories. If you’re serious about Perl, you probably should own a copy or three.

Java in a Nutshell, I had checked out from the public library over a decade ago wanting to brush up on how the language has evolved since my study, and really found its explanations wonderful. Especially if you’ve ever wondered why the answer to Generics in Java is so often no. The only Java  book on my shelf, by contrast was written while JDK had yet to reach a 1.0 release. Needless to say, I had needed updating, lol.