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

Njoy, -- Arun Khan (I have no commercial affiliation with the Humble Bundle)