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I read this in the 80's, probably published around the same time.
The book had two principal strands, a group of professional or semi-professional gamers enter a theme park which exists to provide "real life" RPG experiences. They are playing a new "game".
The second strand of the book was the theme parks security chief tracking a murderer and/or burglar, who ends up being one of the players.
The two strands merge when the security guy is inserted as a PC in the game, so he can investigate the group. There is so much riding on the success of the new scenario that the operators are unwilling to shutdown the game for any reason.
I remember the cover had the party on a raft, possibly in white water, a scantily clad female (a magic user PC) was holding a rune inscribed staff, fighting an enormous snake in the river. The POV of the picture was of someone in the water looking up at the characters on the raft.
The beasties they fought were mechanical, the people they fought were actors playing roles.
The "rune etched staff" was actually a computer, which the "mage" used to give instructions to the operators regarding the spell/alteration she was casting.
Another detail I remember is one of the characters being shocked when their faux sword cut actually appears to cut off someones arm, but that actor had lost the limb previously and the gimmick was setup to enhance the realism of the story setting.
It was English language, set in America, decades before MMORPGs were considered possible.