The Upper Case: Trouble in Capital City (Volume 2) (Private I, 2)

Just when Private I thinks all is calm-now that he's cracked the case of 7 Ate 9-Question Mark storms into the office.
Mark is worried. All the uppercase letters are M-I-S-S-I-N-G! But that's absurd. This is CAPITAL City!
Private I is the last letter standing. Will he solve his BIGGEST mystery yet, the UPPER CASE, before it's too late?!
Filled with the same humor, wit, and quirkiness of the hit 7 Ate 9: The Untold Story, comes another laugh-out-loud whodunit.