OK, thanks for the replies.
I think its just a case of tread warily.
Some of the books I have been looking at have no other reviews that I could find. So I expect they are being hyped.
By the way, for international kindlers there is no such thing as a completely free book in the kindle store. The cheapest, for me seems to be $2.30. This seems to reflect the charge international roaming and VAT.
I can get free books elsewhere, at the sites recommended here, but not at the kindle store.