If you like WWII historical fiction (which tend to be my “easy read” go to genre), I’ve read and absolutely LOVED Winds of War and the sequel War and Remembrance. They’re LONG, about 100 hours total between the two audiobooks, but I’ve learned so much about WWII and the author made a solid attempt to remain historically accurate as much as possible.
I find that Modern Mrs. Darcy has good book lists (I especially like her 20+ hour audiobook list and her big fat books for summer reading list), and she reviews most of the books on her book lists.
When I’m considering books I’ve read that have some substance, I think about books I have thought about long after I read them. Maybe not long books, but books that stick with me or I think everyone should read.
Molokai is not a long book, but I think about it a lot and it got me interested in the history of Molokai and treatment of people with leprosy. The sequel is also great.