Did Larry write ANN a letter the day he died ?

Yes he did. Larry knew the truth about what happened and he could not handle the conviction. He wanted to come clean with Ann before he killed himself.

yeap. Ann is someone Larry trust whole heartedly. I believe that the significance of the letter lies within Ann's inaction to condemn the Kellers, albeit knowing the truth. Is she cunning and deceptive? or is she there to drive the plot forward in order for the Kellers to atone for their sin?