Asgard's Heart — Brian Stableford

The gods of Asgard are in deep trouble. If they can't defeat their internal enemies, the starlet in the macroworld's core will blow up, killing trillions humanoids in its various layers. Only one man can save this articifial planet, and he can only do that by duplicating himself. Unfortunately, the software version of himself that's trying to operate within Asgard's virtual space is fighting on the adversary's home ground, and seems to have even less chance of success than the flesh-and-blood version. Even if they both get through, and contrive somehow to save the macroworld from destruction, how can they ever get together again to become a whole man--and at what cost? The thrilling conclusion to a magnificent sci-fi saga!