Mission Earth by L Ron Hubbard is easily one of my fave sci fi series.
All 10 BIG THICK books.
Battlefield Earth was good to,,nothing really like the movie ( which sucked bugs).
Now,,for acrossover with horror,,checkout the first 5 books in the Necroscope series by Brian Lumley.
There were more after 5,,,but the original 5 were the best, and should have ended there.
Its a good view of vampires and where they came from.
Very science fictiony if you ask me.
As for Terry Pratchette,,yeah,,gofrom the first one,,they all get better than the last.
His writting is really amazing,,I feel like I know Ridcully and the city of Ank Morpork personaly.