When I was in college and graduate school, I mastered the art of reading several books at a time. I had too many to read to be able to devote all of my attention to just one book. I still read books that way; I'm likely to have two or three in progress on my bedside table at any given time.
It took me much longer, however, to accept that I didn't have to finish every book I started. In school I had to because I would likely have to write a paper or exam on the book, and the professor expected me to have read them all the way through. I slogged through an alarming number of books as a non-student thinking that I was going to be tested. Then I was reading We Were the Mulvaneys by Joyce Carol Oates, and I just hated the book. I hated the characters; I hated the writing. I dreaded picking it up to read. Then one day I realized it was just stupid and pointless to keep going. So I put the book back in the bookshelf and didn't pick it up again until it was time to donate it to Ella's school's used-book sale.
Right now I have several books in progress that I haven't yet decided whether I'll finish.
One is a biography of Edith Wharton, who is probably my favorite writer, by Hermione Lee. I tried to read Wharton's autobiography, called A Backward Glance, and got frustrated with how shallow it was. She didn't reveal much about her childhood or troubled marriage or life as a writer. It was mostly a recitation of all the people she met and had dinner with. I quit reading when she devoted pages to her car rides with Henry James. So when Lee came out with seemingly exhaustive study of Wharton, I splurged and bought the hardcover edition. And now it's just sitting on the table. Lee is certainly exhaustive in her research; I know lots about where Wharton lived and the books she may have read, but I can't say that I've actually learned anything about Wharton that I didn't already know. Perhaps one day, when I'm out of other things to read, I'll pick it up again.
I quit reading Elinor of Aquitane by Allison Weir out of frustration. Elinor was one of Europe's most powerful women, but so little is known about her. Weir's book largely consists of speculation about what Elinor may have done and where she may have gone. I want first-hand documentation, please. Give me letters, personal accounts, something other than speculation based on items listed in royal accounting books.
I also gave up on Wars of the Roses, also by Weir. She tries to cover too much ground in too short a book and ends up not doing a thorough job on any of it. Plus, those damn Brits keep changing their names, and I just can't keep up. One minute someone is the Duke of X, then he gets promoted or knighted or whatever, and then he's Earl of Q. I can't remember everyone's multiple names, and Weir doesn't help with tracking them at all.
I think those are the only books I've left unread lately. It's a much shorter list than I anticipated. Next up, maybe, the list of books I want to read.