If you liked "Roadside Picnic" you should really watch Tarkovsky's "Stalker" (though the film is much more mystical and less science-fictional than the book).

If you like Lem, I would heartily recommend "Cyberiad".

Unfortunately, Tarkovsky's adaptation of Lem's "Solaris" was pretty awful, especially compared to the likes of "Stalker" and other great Tarkovsky films like "Mirror" and "The Sacrifice".

Curiously, despite Stapledon's enormous influence, to my knowledge none of his books have ever been made in to major motion pictures.