Jack Manley - soldier, adventurer, traveller of the Wild Multiverse and veteran of the All-Worlds War - has quit. After years of endless fighting he has seen too much death, and taken too many lives. But an interdimensional emperor known only as the Warlord has pledged to destroy Jack Manley for his crimes, and even Manley is not certain of his innocence. With the might of thirteen Earths at his command, the Warlord is a danger to all of reality, and Jack Manley must fight through soldiers, dogs with guns in their mouths, manticores, the dreaded annihil-apes and more before he can face him. It's cover-to-cover pulp sci-fi action adventure, as Manley battles his enemies, and confronts his bloody past. The Warlord is coming, and only death can stop him. But how can Manley defeat him without sinking back into a life of blood and destruction?
Jack Manley is a sci-fi book with a touch of parody. I sometimes like to describe it as the result of what would happen if Kurt Russell was cast as the lead in Doctor Who. The lead character rocket-punches a giant dinosaur in the prologue, which is pretty fair summation of the tone.
As for D&D connections, it does have some Manticores in it. There are also a few familiar surnames in their that old-school gamers will recognise.
The book's available here, and I'd appreciate it if you went over and had a look.