As a person who has to use a Screen Reader, math in PDFs is almost impossible to read for me. The problem is almost insurmountable if the PDF is a collection of images, but even if it is a LaTeX-generated PDF, reading anything but the simplest of equations is very, very hard. In these cases, having the LaTeX source to read is a godsend.
To the authors who publish the source of their books: thank you, thank you. I cannot express how grateful I am. To anyone who is related to /working in the publisher space: it would be incredibly useful if there was a process to get the LaTeX source of books upon request, although I understand how copyrights/etc might make this difficult.
Some other books I would like to point out for being open source: Apex Calculus, Open Data Structures.