Complex sentence: Webster sent her more than forty letters between 1827 and 1851, with time passing becoming increasingly familiar with her in his greetings; his last letters were addressed to "My dear, good friend", which was out of character for him.
Split into simpler sentences: Webster sent her more than forty letters between 1827 and 1851. His last letters were addressed to "My dear, good friend", which was known to be out of character for him.