Listed in alphabetical order
Great website featuring interesting articles I find entertaining
Countless clever circuit ideas
I like Bob's book very much. He is also very active on the DIY Audio forums.
Edmond Stuart's website
A great place where experienced audio enthusiasts share their ideas and knowledge.
Chris Russel's website
This looks like a super useful collection of articles and resources. I haven't read any of them, but plan to do so.
Welcome to over 500 posts and to over 50 articles on amplifiers, tube-based preamps, crossovers, headphone amplifiers, single-ended amplifiers, push-pull amplifiers, Circlotron circuit design, hybrid amplifiers, cascode circuits, White cathode followers, grounded-cathode amplifiers, tube series regulators and shunt regulators, the Aikido amplifier, tranformer coupling, DACs and tubes - and hundreds of vacuum tube circuits that use 6SN7s to 300Bs, all explained in careful detail.
Website by DIY Audio member Keantoken. I like his power supply rail filter very much.
Jan Didden's website
Jan Didden's magazine. Haven't read it yet, but believe it is excellent
Haven't read much of this, but it seems interesting
Seems like another Edmond Stuart website
Collection of his ebooks, resources for CFAs, references and regulators
Listed without any order
Spice models from Bob Cordell are well respected in the audio community. I use them almost exclusively for simulating my designs.
This small wiki describes the best setup to simulate FFT with LTSpice
This guide has helped me a lot identifying and solving any convergence issues without resorting to hacks.
Some of this may be very helpful.