What is the function of the "と" particle in this sentence?
沢山手紙を書くからね, と.
沢山手紙{たくさんてがみ}を書{か}くからね, と。
=
『沢山手紙を書くからね。』と。
≒
『沢山手紙を書くからね。』と言{い}った/思{おも}った/書{か}いた, etc.。
This is the quotative 「と」.
Someone said/thought to himself/wrote, etc. "I will write you many letters!"
What the actual action is and who that someone is, only the context will tell.