The Thrawn Triliogy (This basically started the EU that we know of today)Ī C Crispin's Han Solo triliogy (This covers parts of Hans backstory from child to EP 4) Legends (The old canon universe up till the disney purchase in 2015) I will give you some of my personal favorites. IF you would like to check on some books your self I suggest using Wookieepedia Also are you just interested in books or comics as well? Because all Legends comics as well as the NEU comics (on a six month delay) are available online with Marvel Unlimited Also r/starwars has some archived threads about what books to read on its FAQ.
For a lot of good lists check out r/StarWarsEU or r/starwarscanon.