Best beginner DSP book for developing plugins
There are a number of books in addition to Zölzer's DAFX, the one I would recommend is:

The Audio Programming Book by Boulanger...

Also have a look at these:

Designing Sound by Farnell

Designing Audio Effect Plug-Ins in C++ by Pirkle

Designing Software Synthesizer Plug-Ins in C++ by Pirkle