In fact I think you should have both.

Practical mod_perl is very good general coverage, Writing Apache modules gives you the good lower level info on all the various functionaility.

You should also consider "mod_perl Developers Cookbook" by Young, Lindner and Kobes.

I find all three of these are invaluable, I am relatively new to mod_perl programming (but have hundreds of thousands of lines of CGI behind me).

jdtoronto