A few years ago one of my aunts gave me the family copy of the Genealogy of the Sharpless Family (Gilbert Cope, 1887).  The book is not in great condition, so I spent some time this weekend assessing its state so I can order a box to house it. As a result, I spent several hours leafing through the volume one page at a time - something I've never done before. (It's 1,332 pages long.)
I found a number of interesting things inside the book, both its content as well as things stuck in amongst the pages. But one of the things I found most interesting were several entries where someone had annotated in pencil corrections and additional information.
Specifically, the entry for Margaret W(ilen) Sharpless:
The unknown "annotatee" added in a birth date for Margaret, and children for two of Margaret's children, Charles Delany and Rebecca Delany Collins.
Another one I found was the entry for Petera B(rown) Worrall where the name of Worrall's son, Charles Addams was corrected in pencil, and the name "Dorothy" (his youngest daughter) added at the end of the entry.
 Cope, Gilbert. Genealogy of the Sharpless family, descended from John and Jane Sharples, settlers near Chester, Pennsylvania, 1682: together with some account of the English ancestry of the family, including the results of researches by, Henry Fishwick, F.H.S., and the late Joseph Lemuel Chester LL.D.: and a full report of the Bi-Centennial Reunion of 1882. Philadelphia: Published for the Family under the Auspices of the Bi-Centennial Committee, 1887.