Madeleine L’Engle’s The Wrinkle in Time Quartet, which contains the Newbery Award winner, A Wrinkle in Time, and also A Wind in the Door, A Swiftly Tilting Planet, and Many Waters. It’s on sale for $2.99 in the US. This is a great price for the four books, even if you already have one or two!
If you missed the omnibus in the OP, the first book, A Wrinkle in Time, is $2.99 right now at Kindle US, as part of Kindle’s “From Page to Screen” sale, good through 8/21/2022. And it’s price-matched at Kobo US as well.
Kindle US: https://www.amazon.com/dp/B004OA64H0
Kobo US: https://www.kobo.com/us/en/ebook/a-wrinkle-in-time