Techie Books to Learn Wordpress Advance Developement

I know this is Advanced Wordpress BUT I wonder if anyone can help?
Can you recommend a good 'techie' book for someone who wants to learn Wordpress (not me I hasten to add). It is for someone who is happy coding in Perl and understands MySQL, he doesn't do videos so YouTube etc is out. He wants to go and buy a good in-depth book.

Any suggestions?

http://www.wiley.com/.../WileyT.../productCd-111844227X.html

Digging Into WordPress

Pro WordPress Theme Development

Smashing WordPress

Smashing WP is good, Digging Into WordPress is co-authored by Chris Coyier, who can't really do wrong.