An author's books often get read in a different order than when they were published. That's fine--unless they are part of a series, books are meant to stand alone. But as a writer, I'm interested in seeing what came first, what came second. I'm interested in what new ideas the writer tried along his career, what improvement there was.
In reading two books by Katheryn Erskine, I commented that her YA protagonist (Matt)'s habit of not looking people in their faces felt odd, but it fit perfectly for her (written later) middle grade protagonist, Caitlyn, who had Aspergers. In John Green's work, the desire to matter in the world fits so much better as part of Gus's character (The Fault in our Stars) than it does in Colin's (An Abundance of Katherines.). Maybe it's because Colin is moping about the idea, feeling dumped and washed-up, and so this desire sounds like whining. The same desire put on Gus feels like one more extension of his love of metaphor, and it also feels noble. And a bit heartbreaking, given his trajectory.
My poetry professor used to answer the question "What's your favorite poem that you've written" with a cheerful "My next one." Every time he said that, I thought, that's confident. I can't even imagine what my next poem/short story/novel will be. I just poured everything I had into the last one and got it as good as I knew how. And you can imagine the next?
It's a good practice, to remember that writing keeps getting
better. That sometimes the bits and pieces that we're insistent upon
using are waiting to find a home in a story we haven't conceived of yet.
Another poet, whose face I can picture but whose name has sadly escaped me, commented that writers often have a few concepts, maybe 3-4, that move us. We're obsessed about returning to them. (Case in point, Sharon Olds wrote an entire book about her father dying of cancer, aptly named The Father). The idea's going to crop up anyway. Whether it's a certain kind of relationship, or mattering in the world, or giving a voice to those who don't have it, embrace it. It's what you care about; it's what haunt you. Let it show up in your books, even as you keep pushing yourself to new distances in form and content.