JEFFERSON COUNTY, Colo. — This month marks 20 years since 12 students and a teacher were killed at Columbine High School in Jefferson County.  Now, the school’s beloved long-time principal is out with a new book about April 20, 1999 — and the recovery ever since.

“They Call Me Mister De: The Story of Columbine’s Heart, Resilience, and Recovery,” went on sale Sunday and can be found at booksellers like Amazon and Barnes and Noble.

DeAngelis told FOX31 the book is a tribute to the 13 killed at Columbine and the heartbreak the Columbine community has had to overcome.

“Right after Columbine happened, people approached me and said, ‘You really need to share your story.’ And I said, ‘That’s the last thing on my mind right now.  My number-one thing is rebuilding that community,'” DeAngelis told FOX31.

After he retired in 2014, an old friend approached him and the idea for the book was born.

“Originally, people were saying, ‘We don’t need another Columbine book.’ Well, it’s more than just a Columbine book. It pays tribute to the 13 who lost their lives. It pays tribute to the injured and sharing those stories of hope,” DeAngelis said.

He hopes it will reach out to a variety of audiences, including parents, students, educators and law enforcement.

These days, DeAngelis travels around the country telling the story of how his school rebounded from the tragic shooting. The book takes stories from his public presentations and puts them on paper for everyone to see.

“If we sell any books, all of the proceeds are going to go the permanent memorial, and the Frank DeAngelis academic fund and then the Safety Center. I want to donate the money back just to help pay it forward, because so many people helped us,” DeAngelis said.