Ebooks, because I can pirate that way (Kobo Clara BW). That way I can also take a light e-book reader for traveling and not read it instead of taking a heavier book and not read it.

I've been reducing my small paper book collection.

E-mailing my library and asking if they are interested in any and they picked up four (still can't believe they didn't want "Algeria: France's Undeclared War" wtf)
Putting them up on a local auction site (if they aren't already oversaturated with copies of what I have)
For the ones where there are already many copies on the auction sites, I donate them to my library's "give away a book" shelf, where other librarygoers just take them. Apparently the books disappear pretty quickly