There are 6 books in the The Guardians of the Flame series. The newest book is The Road to Ehvenor which came out in 1991.

Seven college students meet for another night of role-playing games, where they will be cast as wizard, warrior, cleric, or thief. However, their conniving gamemaster has something entirely new planned for them. The seven gamers are cast into an alternate world of magic, fire-breathing dragons – and a quasi-medieval culture…
When you buy books, we may earn a commission that helps keep our lights on (or join the rebellion as a member).

2
The Sword and the Chain

The Sword and the Chain
Having passed through the Gate back to Earth, Karl Cullinane, Walter Slovotsky and other surviving fellow fantasy gamers now return to the Other Side, a place of mystery, magic, dragons -- and slavery.

The gamers build their own town, using engineering skills learned on Earth. But their fight against slavery…

5
The Warrior Lives

The Warrior Lives
Jason Cullinane inherits a kingdom upon his father's death and a quest to rid the world of slavery and must battle against the mighty slavers guild while trying to discover the truth about his father