How I hate that expression! It has the same affect on me as canned laughter on a TV programme: it's like an instruction to be amused, especially when it's emblazoned on the cover of a novel, and (for me) immediately removes any tendency to be thus entertained.
I've just tried to read such a novel, twice, but couldn't stomach it at all. The author tried SO hard to be funny, and the more she tried, the more unamused I became. Different sections were almost labeled "laugh here", thus removing any amusement that might have been found in them. Few things are more subjective than humour; one person's funny book is another's dull/pretentious/ incomrehensible read. I really think it's better not to label books in this way at all; if it's funny, people will find out for themselves, and the surprise element - suddenly finding the book one is reading is very funny when the reader least expects it - simply adds to the humour.
I remember, as a child, reading Three Men in a Boat. I didn't know it was meant to be funny, and when I got to the part where "Harris trod in the butter", I wept with laughter. Why? I've no idea. As I said, humour is so subjective. Having said all that, the funniest book I have ever read (fairly recently), known to be humorous, proved to be just wonderful. Once again, I was crying with laughter. Others to whom I have recommended it have been puzzled/bemused, although some have loved it.
So I won't spoil it for you by telling you what it was. But if you really want to know, you can email me...