Hi bluebicycle. I used to have the reading issues you mentioned,but not all of my life. Only during a certain period of the worst years of my illness. I do think that some bipolar medications contribute to that issue. I do. However, even just bad episodes likely contributed to mine. A combination of brain healing and medication changes/reductions improved how I retained information.

The first tip that came to my mind (especially since you're doing pleasure reading only) is to not be hard on yourself about this. I wouldn't struggle reading and re-reading lines, if it's difficult. That can cause anxiety and frustration, which exacerbate things. Instead, skim the book first, and take little notes of stuff (and their page #s) that seemed interesting at first sight. Then go back later, if you wish, and delve into it again a little more closely. Kinda like reading a juicy thing on the final page, and asking "Ooooooow, I wonder what she did to get to that point?" Then search for the answer by skimming through the book in the likely chapter(s). When you find it, it's like a little pot of gold. Then maybe you can enjoy that spot and bathe more deeply into the relevant section(s).