Encyclopedia of Electronic Circuits, Vol. 3

There are 126 chapters covering all sorts of analog and digital circuits. Volume 3 was first published in 1991, so this isn't state of the art stuff, but if you enjoy tinkering with circuits, there is a lot of fun stuff in this book. My 5 star rating assumes that you will be getting it used at a very good price. I think at this point there are roughly seven volumes at this point. You might try your local library to see if they have any of this series. If you like them, as I did, come back here (or to other used booksellers) to find yourself reference copies. If your lucky, you may be able to score several volumes for under $20 each.