I agree with your comments. I currently have an unusual billing problem whenever my wife or I buy a book through iBooks, but we can't even get Apple to respond to our requests for assistance. They issue an automated case number, promise to reply within 48 hours, and then - absolutely nothing! Not even a polite "Sorry, we can't trace the problem". We are good customers, the problem is genuine, and the request are politely worded. Yet we get zero support. Disappointing and really quite odd in a way. Like you, we've switched totally to Kindle and will stay that way unless Apple are willing to assist us.
Could you perhaps post the name of the author that you're searching for? Other people could then try searching and post a link if they have any success. We have several types of Apple devices here (iMac, iPad, a laptop, iPhone and even an old iPod) so it might be interesting to see if they give different results with their versions of iBooks. We're also the other side of the world from you, so we may see regional variations in availability.
Cheers, Chris