BYU's Family History Archives
The Family History Archive at Brigham Young University has digitzed a number of family histories and placed them on their website.
I found one book that I already had used years ago, but it is nice to know I can easily access it if I want to refer to pages that I did not copy originally.
A clip from page 19 of the DeMoss Family in America is a part of this post (John DeMoss born 1718 is my ancestor). Of course, be certain to doublecheck anything you find in a published family history. These are excellent resources, but they are still compiled sources. The images are full-text searchable and pages can be viewed using Adobe Acrobat. No membership or account is required to use the site.