There’s usually one or two bundles a year that give you plenty to read. I have so much content from the last few years.

There are so many entry points that if you don’t have a very specific desire it doesn’t matter a ton where you start.

The Humble Bundle is a solid deal. The Tome of Fire trilogy is a big Salamanders line. The rest is pretty standard current space marines stuff. If you’re looking for backstory, specifically Dark Angels, tracing their path through the Horus Heresy novels is a good idea. If you’re looking for non-marines starters, Eisenhorn and Ciaphis Cain are good starting points. I am personally fond of the Vaults of Terra trilogy (have not been able to get into the sequel yet). There’s only a few Necron novels so if that’s your thing it’s pretty easy to start. I don’t know a ton about the other races and their fiction.