I doubt I'll ever do another book collaboration; I've been spoiled. Roger and I both happened to move to New Mexico at about the same time, when we each had a family of young kids to raise. Socializing seemed to lead naturally to working together.

The Dracula Tape (Warner June 1975) / (Ace Jan. 1984
"Deathwomb" Poul Anderson (nv) Analog Nov. 1981) (short fiction collection; only 2 original/uncollected stories); (available online) as a Baen Free Sample from Berserker Death omnibus

Berserker Base (Tor March 1985); Mosaic Berserker novel with several guest authors contributing original stories; Saberhagen wrote the overarching story in segments between them using the Niven story as the novel's fulcrum point:
"What Makes Us Human" Stephen R.

Saberhagen also wrote a series of vampire novels in which the vampires (including the famous Dracula) are the protagonists and a series of post-apocalyptic mytho-magical novels beginning with his popular Empire of the East and continuing through a long series of Swords and Lost Swords novels. F.