I have many collections on my e-reader, most of them organized by month. As I've mentioned before, I schedule my reading for review by the month the book is released. If something is coming out late May or early June, it's in the “June 2013″ folder, and if it's late June, early July, it goes in “July 2013.” I do this in case I get so excited about a book that is So Awesome OMG YAY, it won't be too early for me to write about it (I know it makes you bonkers if there's epic squee about a book that's not on sale yet) and it won't be so far in advance that by the time the book comes out, I don't remember what it was about (There was a dude…. and stuff happened? Yeah.)
Aside from the calendar folders, I have a few folders of books that live on my Kindle full-time. They're permanent residents of my reading collection, and if I can't figure out what to read next, I go diving into one of those folders and enjoy something I've already read, or pick out the next book in a series I'm trying to savor. Among my permanent resident folders: Kate Noble, Julie Anne Long's Pennyroyal Green series (I've picked up many of them on sale and am now trying to slowly read in order), and Patricia Briggs' Alpha/Omega series. I also have a “Historical Anytime” folder of historical romancees (obviously) that I've picked up on sale, and a “Novellas” folder of the same. If I'm mood reading, I often read historical romance.
What about you? Do you have books that live permanently on your e-reader? And if you don't read digitally, do you stash favorite books in secret locations just in case you are caught without a book to read? What books have permanent-resident status for you?