From Internet Genealogy's free weekly newsletter:
Many useful 19th and early 20th century Civil War regimental histories are so rare that they’re difficult or very expensive to find.
Many of these works are in the public domain, and are available for free viewing and downloading through Google Books, books.google.com and the Internet Archive, archive.org.
New volumes are being added to these sources, so check back on them now and then.
By David A. Norris, Author of Tracing Your Civil War Ancestors
(submitted by Lynne Parmenter)