The Secret to Strength

 is the second volume of the OnePunch-Man manga series.

Chapter list

 * 10th Punch: Modern Art
 * 11th Punch: The Key To His Power
 * 12th Punch: Paradise Group
 * 13th Punch: Speed
 * 14th Punch: Don't Know You
 * 15th Punch: Hobby & Work
 * 16th Punch: Passed The Exam
 * 17th Punch: Sparring
 * 18th Punch: Business Activity
 * Volume 2 Extra

Release info

 * Release date: December 4, 2012
 * ISBN: 978-4-08-870702-0