a book that is among those having the largest sales during a given period.
any product that among those of its kind is selling particularly well at a given time:
This car was a bestseller last year.
a book, record, CD, or other product that has sold in great numbers, esp over a short period
the author of one or more such books, etc
Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
The book was a runaway bestseller the moment it appeared.
Not yet a bestseller, this song was well on its way last week.
Read something while you wait for that hot, new bestseller to come back in.
The novel, a fictionalized account composed from letters written home during the war, was her first bestseller.