Madeleine L'Engle's A WRINKLE IN TIME was rejected 26 times

Did you know that Madeleine L'Engle almost gave up writing when she turned 40 after getting yet another rejection notice? "With all the hours I spent writing, I was still not pulling my own weight financially." She discovered, however, that her subconscious wouldn't let her NOT write. 

"I had to write. I had no choice in the matter. It was not up to me to say I would stop because I could not. It didn't matter how small or inadequate my talent. If I never had another book published, and it was very clear to me that this was a real possibility, I still had to go on writing." (Source)

A Wrinkle In Time was rejected 26 times before John C. Farrar of Farrar, Straus and Giroux published it. It ended up winning the 1963 Newbery Medal and became a beloved classic.

Sources:

The Storyteller: Fact, Fiction and the books of Madeleine L'Engle - by Cynthia Zarin on NewYorker.com

Awards & Honors: 2004 National Humanities Medalist, Madeleine L'Engle

Penguin Random House Audio Publishing page about Madeleine L'Engle

Wikipedia pages on A Wrinkle In Time and Madeleine L'Engle (though I notice a lot of conflicting info!)