On the experience of reading the same book at 20 and 45: the book is the same. You are not. This is the most important fact about rereading and it's obvious once stated but takes years to fully absorb.

The books I most regret not having access to at specific ages: Stoner at 40 — it would have been a different book at 25, more abstract, less personal. Marilynne Robinson's Gilead at 35 — I read it at 22 and thought I understood it. I did not understand it. The Great Gatsby at 15 — I was assigned it at 15 and missed everything. At 30, I finally understood that it was a tragedy not about love but about the specific American delusion that the past can be recovered.

The books that wait for you are the books worth keeping.