All the kid mystery series--Nancy Drew, Bobbsey Twins, Trixie Belden (but not Hardy Boys for some reason--they were too dry? And no girls!) I remember the wait-Santa-isn't-REAL?! letdown of learning that "Carolyn Keene" was not, in fact, a real person, but that the books were only packaged. I've had a healthy suspicion of packaged books ever since. (Although yes, I do realize how important those seemingly churned-out series are in the process of reading for kids! My kids all went through levels of those, too.)
The first publisher I was ever aware of was Farrar, Strauss, and Giroux, because of their cool logo on A Wrinkle in Time. Which I read over and over and over again. (They also gave me my first ever personal rejection, so they continue to hold a fond corner in my heart.)
Another book I looooooooved in elementary school was Jane Langton's The Diamond in the Window. A New England transcendendalist fantasy treasure hunt. Also, anything by Roald Dahl (I just read three of them to my 6YO, who loved them). And Narnia, which no one told me about, but which I discovered in a Scholastic book order. Also The Lion's Paw by Robb White, a very old book my mom read to numerous school classes, and then to us kids, too, about some runaways in Florida during WWII. The book is actually quite well written and my own kids liked it, too. And The Ghosts, by Antonia Barber, which I read so many times in class that my 5th grade teacher finally just gave me the book.
Er, I'll stop now...
