ebooks are usually much cheaper than physical books

Rarely true for secondhand older books, usually true for newer books or relatively rare older works (which unfortunately includes many academic books). I can often pick up secondhand books for literally pennies; I'm a newcomer to e-reading but not convinced yet that it's going to bring savings overall.

ETA: in case that's not clear, I think this post is missing a huge tip for efficient acquisition of words: secondhand physical books. It's worth saying because some people - I used to be in that number - have a hangup about buying used books. I've totally changed my mind on that, largely thanks to Amazon Marketplace. Riffling through stacks in a used book store holds no appeal for me, but looking up some title that looks interesting and seeing a copy on Marketplace for a euro or less, and buying it without even a second thought? Pure bliss.