I've come to a startling conclusion: what makes a good book is completely subjective. (gasp!)
What makes a book good to me won't be what makes a book good for you. But I'll still give you my opinion.
What makes a good book for me? Great characters.
I can't tell you how many books I've grabbed off the shelves, thinking the premise was great, the characters sounded great and together... they turned into either a boring or not believable read. I hate when this happens, but reading books that I thought would be good and having them turn into door-bangers has taught me one thing: to create characters I believe in for my own books.
Defining great characters is a bit more difficult. For me, a good character is believable. They may do fantastic things, have fantastic cars/homes/jobs but they are still people that as you're reading turn into people you can see yourself talking to. Maybe they have traits (the way they talk/think; the people they have relationships with; unique idiosyncracies) that you recognize from people in your own life or relationship circle and that draws you to their "book person". Above all, for me, great characters are people. They stop being someone you're reading about and instead become someone you're making a journey with. I'm starting to sound like Oprah, I know, but if you can't identify with the characters in books, what are you identifying with?
My CPs tell me I create solid, believable characters and I think so, too. Sometimes I want to throw my characters out the window and close their book, but I work through the anger.
So, by my own definition, I should be writing books that I would buy if I was a random shopper. Does that mean my books have a snowball's chance in Jamaica of making it to the best seller's list? Haven't got a clue.
Will this make me stop writing the best books I can? Absolutely not.
And on a different subject: I hit my summertime goal two days ago. The WIP is done ... needs a bit of spit and polish (the spitting by the CPs, the polishing from moi), but I'm way excited about it. I love the characters, I love the setting....I hope, once it's fully ready to go, that an editor loves it, too!