Here are two important educational analog reference books from in PDF format.

"These old publications, from 1966 and 1963, respectively, are some of the finest works on op amp theory that I have ever seen. Nevertheless, they contain some material that is hopelessly outdated. This includes everything from the state of the art of amplifier technology, to the parts referenced in the document - even to the symbol used for the op amp itself:"

Handbook of Operational Amplifier Applications - PDF
by Bruce Carter and Thomas R. Brown - sboa092a.pdf - 2.1 Mb

The operational amplifier is an extremely efficient and versatile device. Its applications span the broad electronic industry filling requirements for signal conditioning, special transfer functions, analog instrumentation, analog computation, and special systems design. The analog assets of simplicity and precision characterize circuits utilizing operational amplifiers.

Handbook of Operational Amplifier Active RC Networks - PDF
by Bruce Carter and L.P. Huelsman - sboa093a.pdf - 3.6 Mb

This is a handbook on active RC networks. The first question about this subject that one might ask is, "What is an active RC network?" The answer is simple. It is collection of resistors, capacitors, and an active element (or elements). Viewed in another sense; it is a circuit without inductors. Why leave out inductors? There are many reasons. First of all, the inductor is a relatively large and heavy element. This is especially true at frequencies in the audio range and below.