Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
letter [ˈlɛtər] n (correspondence) письмо́#; (of alphabet) бу́ква
small/capital letter строчна́я/прописна́я бу́ква
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: