The Abbot Collection at Houghton: General
Harvard's Houghton Library apparently contains some 10 linear feet of boxes holding the papers of Gen. Henry L. Abbot. Houghton is open to Harvard-affiliated people and other researchers by special permission, but is not open to the public at large. This author got in with a card available to alumni. I do not know how hard it is for a non-Harvard-affiliated person to get permission to use the library. Here is the library's webpage.
The library's reading room is a strictly controlled environment. No containers (from glasses cases to briefcases) are allowed, pens are forbidden, and loose papers must be inspected and identified. Items are called from the stacks via an on-line system and can be reviewed in the reading room. Laptops are allowed, and a limited number of hand-held photos (listed on a special form) can be taken as a note-taking aid. Publication-quality images of materials can be requested (via another form) from the Harvard Library graphics unit. The librarians are numerous and very helpful.
Here is the Library's Guide to Abbot's papers. There is not, however, a correspondence between the Item numbers (in parentheses) of the items in the Guide and the numbers of the Boxes on the shelves. For instance, the two volumes of letters and papers (volumes "A" and "B"), the indexes to which are shown in the Image Gallery on the following page of this website, are found in Box 10 of the collection, but are listed in Series I as Item 14 in the Guide. Box 10 also contains the photos listed in Item 15 and the "first" Item 16. Clarifying up the cross-referencing between the Items in the Guide and the Box numbers on the shelves would be a lengthy process (which has not yet been accomplished in cataloging the collection). Thus far I have only looked at Items out of Series I in the Guide.
The "diaries" noted in Series I, Item (1) are little pocket diaries, generally about 4x6 inches or so in size. The guide says there is one for each year from 1856 to 1919, except for 1896. These are kept in Abbot's very precise (but often tiny) script and the ones I looked at (15 or so) all seemed to have at their end a detailed accounting of income and expenses for each day of the year, with monthly subtotals drawn so that Abbot could track his cash balance.
Some of the books of correspondence contain what look like hand-written copies (via carbons or another type of ink-activated (brown text) tissue paper) of Abbot's letters; these can be very hard to decipher. Othe letters are typed. The letters he received and his replies are often filed together, and some of the letters he received are glued or bound into the letter books. There are also books that contain many ephemera--ribbons, event programs, printed invitations, and the like. It looks like Abbot saved everything.