Beginning Web Development with Python: from prototype to production with flask, tornado and nginx by Andrei Dan
Building Web Applications with Flask by Italo Maia
Data Visualization with Python and JavaScript: Scrape, Clean, Explore & Transform Your Data by Kyran Dale
Flask Blueprints by Joel Perras
Flask By Example by Gareth Dwyer
Flask Framework Cookbook by Shalabh Aggarwal
Flask Web Development: Developing Web Applications with Python by Miguel Grinberg
Instant Flask Web Development by Ron DuPlain
Learning Flask Framework by Matt Copperwaite and Charles Leifer
Mastering Flask by Jack Stouffer
Python in 24 Hours, Sams Teach Yourself (2nd Edition) by Katie Cunningham