No, Seals are part of the Navy, Delta Force is part of the Army They can if they choose not to continue being a SEAL when it is time to reenlist, and enlist in the Army Delta force, but then they would be Ex-Navy SEALs. Besides, SEAL's dont go for Delta Force, they go for DevGru.
Yes. There is a page about Delta Force in Wikipidea. There are countless books about Delta Force, some are written by former Delta Force operatos. There is a book called 'Delta Force', written by the first Commander of Delta Force, Col. Charlie A. Beckwith(Ret).
Yes. The actor Liam Neeson does appear in the film called The Delta Force. He plays the role of a Delta Force member in the film, but is uncredited. The Delta Force was released in April 1986. And it was directed by Menahem Golan.