Seth Godin's Big Head.
Brevity is next to godliness in my book and Seth Godin's latest offering The Dip scores on that front. It can be read on a very short train journey (and was) but it also bears repeated readings, not because it's complicated but to remind you of the basic message at its heart. It even has some plagiarisable cartoons.
Unlike some reviews that bizarrely seem to have read its encouraging tone and its "when to quit" theme as a self-help tome, I see it as rooted in long tail thinking. The long tail is not just about retailers aggregating small unit sales into a big business, it illustrates the explosion of the marketplace into a multiplicity of niches - each of which can be seen as a sales curve with a big head.
Each head is highly lucrative, but each curve is now made up of increasingly informed and demanding customers. If you don't over-deliver to them, they will ensure that you never get out of the dip and if you can't work out how to get out of that dip, then you should quit and look for a niche to which you're better suited.
Because it's a deliberately small book, readers will have no excuse for not having time to grasp that powerful concept and all the more time for the really difficult task of working out how to be the first person in their world to achieve it.