Yeah,
One single book of any RPG it would definitely be Hero 5th ed. Revised (6th ed. is better but is now 2 books).
If xD&D can get away with PHB/DMG, I'd say HERO can get away with its two-volume core set.

Either edition, though--and even the original 4th Edition HERO System Rulesbook--is arguably among the most complete RPGs in existence. As Lord Mhoram said upthread, after you've got the core, the rest is largely examples.
For xD&D it would have to be the Rules Cyclopedia. (If only they had made one volume compiling all rules from the boxed sets and one volume for all the Mystara Gazetteers, but as it stands it's the best self-contained game). Even if you went to 2 books for D&D this would be it as it is a PHB, DMG, MM, and setting all in one!
Seconded. I have at least three 'core rules only' RPGs that come to mind as ones I could be eminently satisfied with:
If it were 'one game system, core rules only, on a desert island forever', HERO 6E (or 5th if you're going to be unfair and limit us to one-volume rulebooks while letting xD&D get away with the two-volume set.

)
For D&D, the Rules Cyclopedia. (2E is too clunky for me without the option to cherry-pick from supplements and streamline via houseruling, and 3E/3.5 look to require nearly as much work as HERO without as much flexibility.)
For Star Wars gaming, Star Wars Saga Edition.
Honorable Mention for if I wanted something faster and lighter would probably go to my copy of BESM 3E.