A friend of mine once said that she never bought me books as presents because it was too hard - there wasn't one particular type of book that I liked. My response to this (in trying to think of what all the books that I loved had in common) was to say that I liked books about 'proper people in interesting situations'. You may think that this encompasses all good books in every genre, but you'd be wrong.
In a further discussion about this very topic, my friend then commented that the worst 'commercial/mass-market' novels were about 'two-dimensional people in interesting situations' and the worst literary novels were about 'proper people in boring situations' or indeed sometimes even 'proper people in no situations at all'. Sad, but unfortunately true. So, just to give you two very personal and subjective examples from opposite ends of the literary spectrum:
1. The Da Vinci Code by Dan Brown: Cardboard cut-out people in undoubtedly interesting, but utterly ridiculous, situation.
2. Heart of Darkness by Joseph Conrad: Proper person in really quite boring situation.
So you see, it's not as easy to achieve as you might think. But what I really meant by my definition I suppose was books which are primarily about people and human relationships. To give you an idea, here are a few archetypal examples of books in my very own invented genre: Revolutionary Road by Richard Yates, Seven Types of Ambiguity by Elliot Perlman, anything by Philip Roth, a book I've just finished called The Master Bedroom by Tessa Hadley, Twenty Thousand Streets Under the Sky by Patrick Hamilton. Does this give you some idea? Of course, it's all very subjective. On the topic of 'interesting situations', I read a few reviews which described The Master Bedroom as 'slow', and I know more than one person who gave up the Elliot Perlman claiming that 'nothing happens'. Likewise on 'proper people', I love Dickens and I know that many would question the 'proper'ness of his characters. So clearly there's no accounting for taste, but we all have to make our own reading rules.
And sometimes, I try to stick too rigidly to these rules. I recently commented to a friend that I was worried about Tim Winton's Breath being 'not for me' - the combination of its setting (Western Australia) and its cover image (a large wave) led me to voice a concern that it would be a book about rugged people doing outdoor pursuits and not chatting about relationships. I was wrong. It's an excellent book and I loved it, and it's definitely about proper people in interesting situations. (Though they don't chat much about relationships, I was right about that, and you can't help feeling that more chatting may have helped some of them significantly.)
So sometimes I'm wrong, and I'm willing to be flexible. But we all have to draw the line somewhere. Ghosts are not usually acceptable for me, unless I can reasonably pass them off as being symbolic or metaphorical. Likewise, I'm not keen on things that can just never ever happen: see, for example, The Time Traveller's Wife. My argument about the books I love and which others claim are actually science fiction - The Road, Never Let Me Go, The Pesthouse, The Handmaid’s Tale and so on - is that they are horrifyingly plausible. We hope that these things will never happen, but they might. In the case of Never Let Me Go, similar and arguably worse things have already happened.