Dear Aunt Debbie,
Rachel's question is an interesting one. I haven't ever sat down and worked out hard and fast rules about this subject, but looking at my own history as a reader, as a child who was read to, and now as a parent reading to my children, I notice a few choices I've made, largely subconsciously.
Some of the first chapter books I wanted to read to Eleanor were series that I remember my parents reading to me: Narnia and Doctor Dolittle, in particular. My memories of these books, and of the intense, talking through the plot experiences I had with them, are still vivid. Reading them to Eleanor has been amazing.
So books like these are fond memories, but they're also packed with issues and ideas that can be teased out with the adult reading them; they're books that benefit from conversation and processing. The discussion of death in the Narnia books, for example, isn't something I'd want my kids going through on their own for the first time.
Even when Eleanor and Isabel learn to read, there will be books at a slightly higher reading level than they can, or will want to, pick up on their own. Some of these are classics -- a little harder to get into, at times. If it weren't for Grandpa, your father, reading me Mistress Masham's Repose, or Treasure Island, Kidnapped, and David Balfour, I don't know that I would ever have picked them up. His reading aloud of these adventure stories brought them to life for me, and strengthened the connection between us.
Some books are just better, and funnier, when read aloud. As you've written, we've read a lot of James Thurber aloud in our family over the years, and especially when shared communally, he makes me laugh so hard I cry. When Grandpa was losing his memory towards the end of his life, I'd read him Thurber stories. Even when he couldn't remember the plot of the story from the beginning to the end, he appreciated the construction of every individual sentence -- the language is so so good aloud. Years before that, I read aloud Mark Twain's The Adventures of Huckleberry Finn to Grandpa and Grandma, the summer I was 13 years old and she was dying of cancer. It was one of my favorite things about that summer. Twain lifted us all out of ourselves.
Many of the books I remember reading most voraciously on my own are those I've written about in our long discussion of good chick-lit (here, and here, and here, among other places). Books with a little more romance or even sex in them than might be really comfortable to read with your parents, books packed with dialogue, quick good reads that don't necessarily shine as literary paragons. I remember going through an entire series of Noelle Streatfeild's Shoe Books (Ballet Shoes is the first). Though I loved them, I'm pretty sure that they wouldn't have worked the same way as read-alouds -- on my own, I could whiz through them and not worry about similar storylines boring anybody.
I guess I'm ultimately not too worried about forcing my loves on Eleanor and Isabel, or spoiling their reading experience of anything in particular. There are always things I'll want to share with them, and there will be things I adored that they'll reject; there will be things they find to read on their own that I won't love as much as they do, and things I will. There will never be a lack, thank goodness, of books.