I'm not sure specifically as the phrase is rather ambiguous. It could potentially be about a set of letters that are small or 'wicket' in some sense, perhaps letters that are part of a mystery or a personal account.
Well, without more context, it's hard to say exactly. 'Wicket' could also be a misspelling of 'wicked', in which case it could be about some rather naughty or mischievous letters. The 'true story' part implies that it's based on real events. It could be a story about a series of letters that caused some sort of scandal or upheaval in someone's life, like letters revealing a long - held secret or some sort of betrayal.