I am reading today Washington In Lincoln’s Time, by Noah Brooks. Most scholars work with the edited version, published by Herbert Mitgang in 1958. Mitgang’s book has some useful notes and pictures, but I am working instead with the online version of the original version, published by Century in 1896.
Why? The INDEX.
According to the index to Mitgang’s book, Stanton appears on ONLY 5 pages: 36, 37, 219, 274 and 278. With the online edition, I can search for the word Stanton, and see that the word appears on about two DOZEN pages. Now some of those mentions may be trivial, but an index should not distinguish in that way; it should pick up every mention of a person or place or organization.
I understand why indices are so often weak. They are prepared at the last minute, when the book is ready to be published, and rushed together in a few days. It is easy to miss a reference or two; indeed impossible NOT to miss a few references. I worked hard on the index to Seward but I am sure there are a few similar issues, ie, missed references.
The moral: when working with an older book, look to see if it is available online, through archive.org, and use the search function to create your own working index. When working with a newer book, look to see if it is available on amazon.com, and use the search function again to supplement the index.