There is a humble bundle deal for O'Reilly Unix and Linux books. For
US$15 you get a whole bunch of O'Reilly Unix/Linux e-books titles. You
can carry them along on a pen drive :)
They are DRM free but respect the author's and publisher's copyright.
Please don't put them up in public; instead make a purchase and
choose where your money goes.
Read more at <https://www.humblebundle.com/books/unix-book-bundle>
-- Arun Khan
(I have no commercial affiliation with the Humble Bundle)