I can attest to both of those books. Audio Programming (Boulanger) has been a great reference, and I recently got sent a copy of Designing Audio Effect Plug-Ins in C++ (Pirkle). It actually covers quite a bit, and the DSP explanations are fairly straightforward I thought (at least the parts I’ve looked at). Rail is right though, its all using the authors platform called RackAFX, which doesn’t get me all too excited (although it is designed to make making generic GUIs easy, with different goals than JUCE). I can see how it would be useful in a classroom setting (which his intro described as his motivation), where the time spent setting up plugin params, and the GUI (and 2-way communication and automation) can eat away useful time from DSP, algorithm, and programming design.
His tool does VST/AU wrapping of his “RackAFX”. In any case, the beginning chapters were easy to read and pretty good, so although its a bit odd he doesn’t mention Juce, which would be great in a chapter to discuss working outside of RackAFX, I think many could still get useful information from the book.