"Surprise bestseller" refers to a book or product that unexpectedly becomes very popular and sells very well, despite not being anticipated or predicted to do so. Full definition
Kobo expects 2012 to bring additional surprise bestsellers from self - published authors, whether through direct relationships or aggregating partners. (goodereader.com)
A featured speaker was the education scholar E. D. Hirsch, whose surprise bestseller Cultural Literacy had appeared a few years earlier. (educationnext.org)
Carlo Rovelli's brief book was a surprise bestseller in his native Italy, and the English edition shows us all why (newscientist.com)