I like organizing mine alphabetically by author. If the author has 1 or more series, the series is organized in reading order. Which is not always published order, or book order. (Valdemar, Pern, Vorkosigan series’, I’m looking at you…) And series I like best get put first under that author.
This tends to only work when I first move into a house or apartment. After that, it’s too much work to shift books from one shelf (or bookcase!) to another every time I buy or am given new books. I’m up to 10 bookcases now, though only 8 are completely full. 1 is half full, and one has both my ceramics and my TBR shelf. Plus a half-shelf of books I’m getting rid of.
I’ve considered organizing by favorite author, or favorite series instead. And then doing anything else alphabetically by author. But I find new series so often now, I’d have to re-arrange even that type of organization.
It would bug me too much to have books just stuffed wherever, in whatever order on my shelves. And while I’ve seen some people have two rows of books on each shelf, I don’t want to have to unpack the front of a shelf in order to get to the back row.
I do separate hardbacks from paperbacks. Unless it’s part of a series. All my series books get kept together even if it makes the shelf look a bit odd with paperbacks, then a hardback, then more paperbacks, or vice versa.
If I don’t keep series together, I tend to forget I have a particular book and go buy it again. Not that I don’t do that already, but it would be much worse!
So how do you organize your shelves? Alphabetical by author? Alphabetical by title? Favorite series or authors first? By book size? By book color? Do you separate hardbacks and paperbacks? Do you organize by subject/genre? And how many shelves/bookcases do you have?