Andrew Owen recently revealed, in a short Facebook post, that U3.5: Pax Britannia will indeed have a cluebook:

Yes, there will be a hint book.

He then clarified its intended content somewhat:

Actually, the hint book is a description of what happens to the locations in the game in between you completing it and the start of U4. The idea is that you can work out what you’re supposed to do from that. Makes up for the fact I don’t have room to fit a really cool ending in the game itself.

And, of course, the cover art was revealed, as can be seen here.