I'd still go the UnrealED, my oldest brother Chris Blundell, started out just making maps with the Unreal Engine, and progressed on to getting jobs through Epic (He did a couple of multi-player maps) for UT2004. And worked on the ill-fated Stargate:Atlantis as a map designer, he also worked on Werewolf: The Apocalypse - The Heart of Gaia, and a maps for Rainbow 6: Athena Sword, as wells as Devastation.
The big plus of working the UnrealEd, is that it has its own level editor (which is really good, and almost real-time in processing lighting and creating the map build!), it has excellent AI path points and is visually very easy to get to grips with, I played with it about four years ago and had no time picking it up. Also there is books on the UnrealEd for creating games which is very cool. Adding in new scripting seems to work well too. Plus there are tons of map-makers out there who would love to be in a mod project. The best idea is to pick scenes out of the book, and do briefs ... how you see them etc etc.
In my brother's words, it is a lot harder to create maps for Half-life 2. Being that you have to make a lot more geometry cuts.
Unreal engine would be my way of thinking. Plus UT games can be snapped up at a fraction of a cost of what Half-Lie 2 is selling for at the moment.
David, there would be no reasons to not being able to keep selling your book, if a mod was in development. Even if you took it to a major software studio, you would need to have that in the contract.
To take it further to the "big boys" you would first need to do a design document. Much like a synopsis ... you could bring to the party with concept drawings, etc etc.
Regarding Flash games, there's potential, but overall the games (in general!), are of limited size. So it might cover a sequence and not the whole book chapters. But then, it may work well, as a "episodes" concept ala Half-Life, Sam & Max or Sin!
The other ideas on the game matter would be to look into DarkBasic, you own the use of the engine, was compiled its a self-run game. And has the ability for 3D applications, and various add-ons (say a character studio) which will import 3D characters from other software (C4d, Max etc etc)