There's some absurd recency effects in textbook publishing. In well-trodden fields it's often possible to find a last-edition textbook for single-digit pennies on the dollar, and the edition change will have close to zero impact if you're doing self-study rather than working a highly exact problem set every week.

(Even if you are in a formal class, buying an edition back is often worth the trouble if you can find the diffs easily, for example by making friends with someone who does have the current edition. I did that for a couple semesters in college, and pocketed close to $500 before I started getting into textbooks obscure enough not to have frequent edition changes.)