Kodak Monographs available for download.
These are very old and out of copyright. Available for free download as PDF.
Kodak Monograph 1 - The silver bromide grain of photographic emulsions (1921)
Kodak Monograph 2 - The theory of development. (1922)
The photographic negative. Very old. Has many interesting chapters however.
(I have no affliation with the above site. It is a well known archive of copyright free materials).