Wiktionary gives billet-doux, which is technically a borrowed word, but is used in the English language in the same manner as any borrowed term, such as crime passionnel.
The only alternative I can think of is the conditional term "Valentine" which can mean "An expression of affection, especially romantic affection, usually in the form of greeting card, gift, or message given to a person the object of affection, especially on February 14th.". While it is conditional, it is the only single word term for a love letter that I could find.
Otherwise it appears that there is no single word term.
As a last note, I would like to point out that the noun "rejection" is not commonly used to refer to a rejection letter; if you have heard it used as such, it was probably being used as a colloquialism or in a context where its meaning was obvious. For example a situation in which a person holding a letter states "It's a rejection". Even in that context, its usage would be uncommon, people almost always say "rejection letter" in full.
There is the word bookish:
fond of reading; studious
and bookishness for the fondness of reading:
the quality or condition of being bookish
The word has other connotations as well but I think this word is the closest single word you can get for this meaning.
As I mentioned in the comments, lectiophilia would be a suitable neologism for the love of reading if we follow the path of bibliophilia (and such words suffixed with -philia). Lectio means reading in Latin. In the end, it can always be used as a nonce word.
Best Answer
I don't think there's a single word, but you could certainly say opened her heart, or shared/revealed her love/feelings.