If you want an in-depth understanding of LaTeX you can start somewhere else.
However, if you want to really understand TeX then there is no way to avoid the great TeXBook! (I actually ordered the whole 'Computers & Typesetting, Volumes A-E Boxed Set' just two weeks ago. It's still on its way to Europe -- the US prices are so much cheaper :-) )
It explains a lot of how the TeX engine works. I really just started to understand some of the fine mechanics of it after reading this book.
The sources of The TeXBook are on CTAN, so you could have a peek in it (in source form only). There is also TeX for the Impatient as a full PDF document. However, I didn't read that book yet.
No, you cannot.
All the books in Computer and Typesetting are copyright of Addison-Wesley and cannot be distributed without permission. Knuth, however, has made all the sources to these books and his programs freely available for people wanting to learn how to use TeX and METAFONT. However the sources (of The TeXbook and The METAFONTbook, at least) make it clear that you are not allowed to try to make the PDF yourself. If you do, you are greeted with an error (and an endless loop):
This is TeX, Version 3.14159265 (TeX Live 2019) (preloaded format=tex)
(./texbook.tex
! This manual is copyrighted and should not be TeXed.
Volume A, The TeXbook is copyrighted by Addison-Wesley and only the source is freely available for direct reading. The very first lines of texbook.texsay:
% This manual is copyright (C) 1984 by the American Mathematical Society.
% All rights are reserved!
% The file is distributed only for people to see its examples of TeX input,
% not for use in the preparation of books like The TeXbook.
% Permission for any other use of this file must be obtained in writing
% from the copyright holder and also from the publisher (Addison-Wesley).
Which means you can read the source, as learning material, but you cannot run tex on the source. As a matter of fact, you get an error if you try to do so.
Volume B, TeX: The program, is distributed in PDF (only the documented source) and you can access it with texdoc tex, and the source is available from CTAN.
Volume C, The METAFONTbook is released under the same license as Volume A. At the beginning of mfbook.tex you see:
% This manual is copyright (C) 1986 by the American Mathematical Society.
% All rights are reserved!
% The file is distributed only for people to see its examples of TeX input,
% not for use in the preparation of books like The METAFONTbook.
% Permission for any other use of this file must be obtained in writing
% from the copyright holder and also from the publisher (Addison-Wesley).
and you also get an error if you try to tex it.
Volume D, METAFONT: The program, is distributed in PDF (only the documented source) and you can access it with texdoc -l mf and then selecting mf.pdf, and the source is available from CTAN.
Volume E, Computer Modern Typefaces, is distributed scattered among the sources of the Computer Modern fonts (CTAN page). I don't know if there is an available PDF version of this one.
Best Answer
AFAIK no. The only authority here is the publisher; please ask this question there, although I have not found an easy way to address that question on their website...
:-(Edit: I personally doubt that DEK is satisfied with the typography of ePubs (which is essentially XHTML & CSS) and would demand PDF - and since the publisher(s) would then probably require DRM (which I presume DEK detests the same as most of us), I consider ebooks of DEK's works unlikely. Also paper lasts longer than ebooks.
:-)