The Martian is a really fun, easy read. It was originally written as a series of online blog posts so it's pretty consistently entertaining without any big, boring filler chapters.

Personally though, I don't read a lot of fiction. There's also a ton of really entertaining nonfiction out there. I'd particularly recommend Isaac's Storm by Erik Larsen. It's about the devastating hurricane that leveled Galveston, Texas in 1900 and weaves in the history of hurricane prediction in a really interesting way, telling the stories of some of the people involved.

I dunno if that will turn out to be something you like, but just a little something different to consider trying.

(Also, I know everyone is saying library and they're definitely right you should do that. But also I wanted to share my favorite eBay book seller as another option for low cost books. Their selection is amazing: https://www.ebay.com/str/secondsalecom)