I dunno, I find it hard to believe that Dean would choose a book named “Perl Primer” as the one Perl book to keep on the shelf if he had many to choose from. As a former Perl programmer, I think he would have chosen the so-called “Camel Book” if he chose anything. (https://en.wikipedia.org/wiki/Programming_Perl) It’s by far the best-written book about Perl, and even one of the best-written computer books in general. I would also expect to see more of those O’Rielly “animal books” for other languages.

My theory is that he got all those books for free because the local university library was throwing them away.