On my Reviews of Flash collections page I have links to write-ups of most of the Flash books I've read. A third of them are anthologies, which is an indication, I think, that it's still an evolving market.
Who buys Flash books? At least with anthologies the contributors might be buyers, but how many of those people will buy single-author books? Who are the trusted publishers?
Reader expectations are maturing now that people are no longer buying only books written by friends. How good should a book be before it's publishable? I think the bar is rising. Should it have sections, like many poetry books have? Will readers accept a mix of prose-poems, dribbles and narrative Flash all in the same book? Are Flash pamphlets viable? Are more Flash books being published by non-Flash-specialist publishers?
At the moment I'd guess that only Flash writers buy (or know about) Flash books. If the customer base expands, are poets or short story writers the most likely additions? Books of short stories and poems aren't flying off the shelves either, but at least the Flash market is expanding. I think Flash might appeal to (narrative) poets who feel that modern poetry (obscure, or "exploring issues") has abandoned them.