I recall listening to an audio cassette version of this book in the mid to late 90s.

Two young boys are invited to try out a new virtual reality game. They are hooked up to some headgear and special suits, with their "game world" a series of treadmills to simulate terrain. They die pretty graphically, with the protagonist's friend burning alive in lava.

After they head back, the two start seeing elements of the game in real life, to the point where they cannot discern the game world from the real world.

The creator of the game explains that the boys have to beat the game and the final boss, a giant, in order to stop the hallucinations.

It was called something like "R_______'s Game" or something similar, I think.