Hello fellow bookworm!
No, in my opinion it’s not a bad thing at all, provided that $90 wasn’t needed for something more urgent such as bills or living expenses. It’s a much better use of money to buy books in my opinion, then spending $90 on a dress or pair of shoes you will only wear once (or never because you forgot you bought it). Books are expensive, so here’s a tip: try to buy used. Most of the time you can get the same books in great condition (just read the description) for a fraction of the cost for a new copy. You could barely tell the difference between the “new books” and the “used books” in my library. Thrift shops, amazon.com, and barnesandnoble.com (marketplace) are some places to find them.