Scarfy
Stephen J Sweeney
To me, those are the worst sort - you feel pressured to buy/read them, but what if they turn out to be (like most self-published books) truly awful?
This is true of any book, though. If you network with a lot of writers, you may find yourself getting on with them well and it's always nice to see them at signings, events and such.
Then you eventually get around to reading their book and don't like it.
So, what do you say?
Simple - you tell them the truth and say it didn't work for you. A shame you didn't enjoy it, but if it doesn't work for you, then it doesn't work for you.
Sure, self publishers might take it a little more personally, because they don't want people to think that their opus is a load of garbage, but so long as you try and be constructive with your criticism, you'll be doing them a favour.