I remember loving this as a kid. Re-reading it as an adult - because of the upcoming movie - I can only say that it's insane. I guess it works in kid-logic, but as an adult it's beyond absurd. Three modern-day kids meet three weird ladies who live in the woods (who turn out to live for millions of years, and bear a strong resemblance to angels in their "true form") who take them off to rescue their father who's been missing a couple years. He's a few thousand light years away, a prisoner on a planet that's succumbed to evil. And the three children (with very limited help from the three weird ladies) are supposed to rescue their father from a power that's subjugated an entire planet very similar to Earth.
Some of the book's lessons are fine, such as "accept your own flaws" (although I wouldn't go too far with that one). Some are less impressive: if you get the feeling someone is trustworthy, they are. And the inverse is also true: if they're creepy or weird-looking, don't trust them. Perhaps this is a product of the era, but these days trusting strangers isn't something we normally teach children. Personally, I'm also not much of a fan of "we are all God's foot-soldiers," another lesser theme.
I'm really struggling with the idea of this becoming a movie (due in March 2018): I'll be shocked if it isn't laughed out of the theatre. The inclusion of Oprah Winfrey does not reassure me.