It seems like you can use this card as many times as you want, assuming you spend the action and are out of skill points. That makes this one of the most powerful starting items. I frustrated the Keeper with my ability to spend skill points like crazy knowing I could get more using this card. Go Jenny!

Skill points don't still win you games by themselves. Try putting monsters in the investigator's space and force them to evade when they want to use the Letter. They'll run out of skill points soon enough and start taking damage for failed evade checks. Plus, if they are using the Letter almost each turn, they aren't really investigating. Don't focus on the Letter-investigator, pick another lamb to slaughter.

Jenny with letter has better base physical stats and ability to regenerate skill points and sanity, BUT has absolutely no means to defend herself. If keeper'll put a pressure on her before she finds any weapon…