Small Signal Audio Design is a great book but I would advice against reading it before you have a very solid background of the fundamentals. Another great one is "The Art of Electronics" by Horowitz/Hill, this book will teach you about electronics in general but it can be overwhelming at times, "Electronic Principles" by Malvino is an easy introductory book about analog electronics like the diode, transistors, transistor configurations, etc.. operational amplifiers; buy a used one, if you want something more advanced, "Microelectronic Circuits" by Sedra/Smith is a personal favorite. For operational amplifiers "Design with Operational Amplifiers and Linear Integrated Circuits" by Sergio Franco and "Operational Amplifiers" by Roberge (which is considerably more advanced) are probably some of the best books on the subject.
For vacuum tubes and much more the bible is "Radio Designer's Handbook" 4th edition, by Smith, this book is a classic but it does require you to know the fundamentals very well so you might want to start with another book called "Valve Amplifiers" by Morgan Jones.
Read application notes, for example Texas Instruments, Analog Devices, ST, etc.. have many great tutorials and application notes. Jensen transformers, Lundhal and Sowter have a lot of literature on transformers. THAT has a lot of material on preamps, line recievers/drivers, VCAs and dynamic processors.
There is a book META somewhere around the forum.
Regarding 500 series projects, instead of trying to adapt circuits to work on the 500 series, I would rather start with some already made projects for the 500 series. This forum has a ton of projects, some users sell PCBs, parts, racks, etc... the search function is your friend.
Welcome to the forum!