I'm sure a lot of folks who don't write think that writing a book must get easier with every one you complete.
In some ways, that's true. Once you finish your first manuscript, you don't have to worry that you won't be able to finish a whole manuscript.
And once you sell, you lose the big worry that you don't have the requisite talent to write a sellable manuscript.
Speaking for myself, the writing itself continues to be a complex, baffling, confusing, worrying, thrilling, exciting process.
One thing that remains tricky for me is deciding the amount of character backstory to reveal in the opening chapters. Too much, and you risk slowing the pace and boring the reader or telling them so much, there's little reason for them to read on. Give them too little, and you risk creating frustration and/or a character who doesn't come across as you would wish. For instance, the rakish hero seems to be no more than an unfeeling, selfish cad because you haven't mentioned anything about why he acts the way he does.
Thus I discover, in the third draft of this manuscript, that I have a chunk of backstory revealed via dialogue that brings the story to a screeching halt. Because of what it reveals, it must be fairly close to the start of the story, but not where it is at the moment.
Like I said, for me, writing continues to be baffling, complex, etc. etc.
But I did mention the thrilling, exciting part, didn't I? That's how I'll feel when I find a better place for that bit.