[Bret L]

Many fine books on audio are now in the public domain and may be
freely downloaded. These do include the classic "Radiotron Designers
handbook" as well as DIY books from several good publishers such as
Howard Sams.

Others are not. The best example of an old book no longer available is
the "Audio Cyclopedia" by Tremaine. Many other desireable titles such
as almost everything by Terman had their copyrights renewed and the
owners want big money so they are out of print. Alfred P Morgan's
excellent kid's books are likewise so encumbered.

Newer books particularly interesting are legion.