That made perfect sense to me, but then I realised that I was stringing two vague, subjective, qualitative adjectives together in a way that really would only make sense to me.
To me, this is simple: a great book appeals across genres, categories and readers. It is offering something different, possibly even unique. And, perhaps most importantly, it has meaningful subtext that has timeless relevance. The great books are the ones we're taught in school, and, in fact can be used as teaching tools: they have layers of meaning.
A good book is entertaining.
I can hear the clatter of toys hitting the floor all over the internet, so let me quickly add: I don't think great is better than good: they're completely separate qualities. Sometimes they even co-exist. I do believe great books are more rare than good books, but, conversely, I also believe that there are more unreadably bad great books than good books.
Examples in fantasy below - using four authors I talk about quite a bit. Again, this totally subjective. I've included these to illustrate the 'model' rather than as prompts to discuss these particular books:
Good (not Great): Brandon Sanderson's The Way of Kings is the perfect good book - entertaining, technically brilliant, utterly devoid of layers or meaning. This is a book perfectly crafted for reader enjoyment.
Great (not Good): KJ Parker's Scavenger Trilogy is a great series. It toys with narrative structure, it questions our relationship with religion, predestination and free will; it talks about how we relate to power structures (familial, political and cultural). It is also, as one of my friends called it, "unbelievably boring". This is a fantasy book predicated on a combat system where the action takes place as rapidly as possible. Not exactly, you know, 'gripping'.
Good and Great: China Miéville's The Scar - I'd be tempted to say Perdido, but The Scar is more of a crowd-pleaser. The Scar is good - it takes the traditional quest model and turns it on its head, and, while giving us monsters, vampires, sword-fights, sea creatures and naval battles. Also it is great, as it leads the reader into layers of discussion about the nature of freedom itself.
Neither Good nor Great: Patrick Rothfuss' The Wise Man's Fear (because, if I've got to pick on a book, I might as well be consistent). This two-axes model actually nails why I dislike this book so very, very much. It tries to be great, but the philosophical layers are, at best, the sort of meaningless tripe you find inside Hallmark cards. At worst, they're outright dodgy. And in the ambition to be great, The Wise Man's Fear fails to be good - it loses the fun and the adventure (and the plot progression) that made The Name of the Wind (a good book) such a joy to read.
What do good and great books mean to you?