As far as I can see, once read, there's no point in hanging onto books or other writings. Should I just sell them to gain money/lose encumbrance? Or are there any books that should be retained?

P.

I don't think the books have weight. I keep them so that I know what books I have read.

They don't weigh anything. But you can sell them for extra oren. Not every Quest Item is unsalable, though. And some notes (not books) contain information for quests that is not in your journal. you also have to open the book to get the journal info, don't forget. So, be careful.

Books you can sell. Other writings, it varies.

You can safely sell them if this is what you wanted to know. Unlike receipts, which are gone from your alchemy menu when you sell them, book texts are automatically noted in your journal. So you don't need to keep them in order to re-read the text.

BTW: I use a mod that makes receipts weightless and adds weight to the books instead. Makes much more sense. ;)