Man, I love this book. It was my first introduction to epistolary fiction, and it's just so adorable. Now the fact that she called her romantic interest Daddy throughout the entire book wigs me a bit, and Jervis is so high-handed and lacking in candour that I'm deeply suspicious of him, but I'm gonna put down Judy's trilling that he's right most of the time because he's years older than her to the mushy-brainedness of the first flush of love, and believe that she's going to be able to handle him. Still, the whole love subplot, Pygmalion and Galatea, is dodgy in the extreme. I like the romance subplot in the sequel-of-sorts Dear Enemy better, but that book has lots of other dodgy things.
Things I didn't like when I read this as a wee kidlet: Jervis? What the hell kinda name is that for your romantic interest? Sheesh. When the only nickname you can come up with is Jervie, you know, you might wanna consider changing the name.
Things I liked back then, and still like now: Judy's resolute independence. Her delight in the books she's catching up on, which everyone else has read: Alice in Wonderland, Sherlock Holmes, etc. etc. The letter about Pepys, because that was the first time I'd ever heard of the dude. The doodles. The digressions on clothes.
Coming to it fresh now I think I'd probably find it a little precious, but I can't imagine not being charmed by Judy's spirit and sense of humour.