A Wrinkle in Time by Madeleine L'Engle (1962)
I hadn't revisited this story since I was very young, and it was a fun adventure with likable characters. L'Engle has created interesting worlds and strange characters, though Meg, Charles Wallace, and Calvin are very relatable kids who experience all the strangeness along with the reader. I still don't exactly understand the tesseract; the explanation of the fifth dimension was hard for me to visualize (and I was never good at science) but luckily it's not integral to following the story.
The only part I didn't like (aside from the numerous religious references peppered throughout) was the really lame simplistic ending. I hate to complain about something being simplistic in a children's book, but I was rather disappointed at the end. I'm sure as a child I found it a revolutionary idea that what defeated the the enemy was love, but as an adult it's hard to swallow. (And sorry for the spoiler - I assume everyone has either read the book or isn't interested.)
There are four more books in the series, and I have no specific plans to read them soon, but eventually I may want to visit again with Meg, Calvin, and Charles Wallace and see what they're up to.