Oh awesome. I used to play alot of dnd. Since 3.5 edition. Played a few others too like cyberpunk, merps and paranoia.
I think one thing that really sets 40k especially apart is the lore. I'd definitely recommend getting into the books. I would personally really recommend giving gaunts ghosts a read. That series is amazing and really will immerse you into 40k
the Horus Heresy books are good but more of an investment timewise and I would say can be hit or miss later in the series.
What in particular interests you in 40k is it lore, painting, collecting, playing? Or all of the above ha