I'm planning to read a mixture of classic 20th century novels, sci-fi (which has some intersection with 20th century classics..) and historical non-fiction. So far I've read one from each of those categories: A Clockwork Orange by Anthony Burgess, The Doomed City by the Strugatsky Brothers and October by China Mievelle.

I like to roughly alternate between fiction and non-fiction. It keeps things refreshing. I also find reading fiction after reading non-fiction makes it easier to glean the ideas the fiction book is trying to convey as you're still in the information processing mindset of reading a non-fiction book. This adds an extra layer of appreciation beyond enjoying the story.

In terms of choosing a book to read - just think of things you've always been interested in and then ask for recommendations for books (fiction or non-fiction) that discuss the topic, or use that topic as a major plot point. A good general rule of thumb I've found is not to bother with books with less than a rating of 4 on Goodreads unless it's a subject you really think is going to interest you, or an author you know you like already.