The Mysterious Girl in the Garden, by Judith St. George--simple time travel for the young middle grade reader
An American girl, Terrie, is forced spend the summer in London, tagging along with her mother to Kew Gardens every day (her mother is studying there). Terrie is sullen and bored; she wanted to stay with her grandma on Cape Cod for the summer instead. And so she is rather deliberately not having a good time. But then a little dog comes out of the shrubbery, though dogs are forbidden in the gardens, and Terrie follows it back, and meets a strange girl named Charlotte, who says she is a princess.
Terrie is naturally skeptical, but soon she realizes that the little dog acts like a messenger to bring her back two hundred years to the reign of King George III. Charlotte is his grand-daughter, and being rather bored herself, she's glad to make friends with Terrie. And nothing much happens except for Terrie visiting Charlotte quite often, until Terrie takes Charlotte's places so Charlotte can go visit her mother for the day (something her grandfather, the king, has forbidden). Terrie almost gets found out, but makes it back to her own time safely, and that's the end.
But though it is slight, it is rather vivid and magical. If you are nine or so and easily pleased by gardens and times past and magic that's totally unexplained and you like little dogs and like to read about girls becoming friends and dressing up in 18th century clothes, you will find it enjoyable. There's no deep point to it; it's basically just a nice thing that happens to happen to Terrie that makes her summer better. Since so few of us are in fact nine any more, it's hard to actually Recommend this any more recommendingly, but I'm happy to have read it; it was nice and soothing on a cold wet December night!
Thanks, Anamaria, for sending it my way!