Frankie Williams is the author of "The Merdragon," a fictional children's book she wrote for her daughter, Shauntae, who passed away last year. Shauntae was only 36 and died from undiagnosed heart disease.

Frankie says, at the time, she tweeted at Shelton and his country singer friend Craig Morgan, because Craig had written a song about his son's tragic death. Blake was promoting the tune, "The Father, My Son and The Holy Ghost" ... and she found it very inspiring.

Frankie says Blake replied, saying he was inspired by people like her and Craig who find ways to help others deal with grief while still dealing with their own ... and it motivated her to write the book as a tribute to Shauntae.

Flash forward to Monday -- Frankie tweeted at Shelton saying she wanted to send him a copy of 'Merdragon' as a token of gratitude for his kind words, and he responded again ... saying he'll order it instead.

And, the kicker ... he encouraged all of his 20 million followers to order it, too!

Frankie tells us ever since Blake's tweet, her book's been selling like hotcakes. She says she doesn't have official sales numbers yet, but it's currently in the top 5 in three different children's categories on Amazon ... skyrocketing up the charts from the 400-500 range it was in before Shelton's shout-out.