I reference this one a lot, http://www.music.mcgill.ca/~ich/classes/dafx_book.pdf, and have enjoyed it. I get lost in some of the math, but it's very thorough and practical. It was also personally reccommended by the guys at Goodhertz, and they seem to know what they're doing.
I'm just a hobbyist, but here's some resources that have helped me. My interests are specifically for musical applications, so that might affect some things.
"The Scientist and Engineer's Guide to
Digital Signal Processing" is free online. This has been my most used text so far. [0]
Introduction To Digital Filters [1]. Another book free online. I haven't dug very deep into this one yet, but select chapters have been helpful so far. Juluis O. Smith III has several other books on the topic that seem useful too.
Earlevel's Wavetable oscillator tutorial. Create a mip-mapped wavetable oscillator in C++. A really nice balance of theory and practicality in both the explanations and code/ [2]
The music-dsp mailing list and the KVR DSP forum [3] have both been extremely helpful. I'd seriously recommend lurking the KVR forum for a bit if you're interested in musical applications of dsp. It's a bit like HN in that you'll ask "how do I replicate this 80's digital synth" and sometimes the original programmer will respond.
Aside from that, blogs specific to your interests. Plugin manufacturers can often get pretty technical on their blogs. I recommend reading through the archive of the valhalladsp blog if reverbs interest you.
And not DSP specific, but working on a fun project is always a good motivator. I started off with VSTs using I-Plug/WDL-OL. Now I'm using a Hoxton OWL (guitar pedal with a Cortex M4f) and have been much more productive than ever, just because the form factor and hardware are so fun.
There's lots of repositories of code examples. AudioTK has a lot of examples and is high quality, I feel [4]. MusicDSP has a TON of code examples, though not all of them are good quality or well optimized, it's like the stack-overflow copy-pasting of DSP, but still useful for the depth. [5] There's probably a hundred others, many optimized for specific hardware, but these are the 2 that come to my mind first.
Edit: Oh, there's also tons of MOOCs and online lectures about this as well. I won't offer any advice there though, as I don't learn well through those.
Meta: what happened to comment favorites? Wasn't this feature recently added? I always come across link lists like this and want to save them, but I'm too lazy/busy to put them in a bookmark manager. Oh well. Nice comment, thanks for the resources. :)
