An excellent find today! I will let the book (or rather, the tipped-in typescript done by the bookseller, which was the Brick Row Bookshop) tell the story:
The Didot autographs:
This interesting pamphlet, published in 1864, besides the fact that it relates to a Parisian family widely distinguished in the annals of printing, the Didots, and besides the fact that it was once in the possession of DeVinne, one of the most distinguished of American printers, has at the end (pasted in), seven manuscript letters of the highest interest, as follows [lists them, but I have provided scans]
The writers of nos. 5 and 6 are the sons of the writer of nos. 1 and 2. In the pamphlet itself, Firmin Didot is mentioned at p. 25, Ambroise Firmin Didot at p. 30, and Hyacinthe Didot at p. 40.