Reddit reviews The Audiophile's Project Sourcebook: 120 High-Performance Audio Electronics Projects (Tab Electronics)

There is a really good book out written by "Randy Slone" called the "Audiophiles Project Sourcebook" It has every circuit design in there that you could want from headphone amps to power supplies to full range or custom equalizers. The circuit designs are very straightforward and there are many different ways to build them. He even lists the exact part numbers you can use. There is a big following of this book so if a part isn't made anymore it's really easy to find the equivalent replacement for it.

I would definitely recommend this book as a must if you're looking to do your own circuit building. Even if you don't use any of the schematics there is a lot of good information on audio design.