Буква means letter, as in a letter of the alphabet, not a letter you get in the mail. It's a perfectly regular feminine noun. When you talk about how many letters are in the English alphabet, there is a consensus that the alphabet consists of twenty-six letters. For Russian the situation is slightly hazier, since over the last hundred years sometimes ё is considered a variation of the letter е, and sometimes it's considered a separate letter. The current standards consider them separate letters, so it's educated nowadays to say that the Russian alphabet has thirty-three letters.

Because we English speakers were taught to worry about spelling, we always wonder how to say that a word is written with a particular letter. In that context Russians do not use the preposition «с», but rather «через» followed by the accusative case:

Of course we also need to know how to say capital letter прописная буква and lower-case letter (small letter) строчная буква. (Aren't you glad you studied a language where each letter has only two forms as opposed to, say, Arabic, where letters can have an initial, medial and final form?)

Notice that in this context the preposition с is used with the genitive case. I think that's because the Russians conceive of it as starting from a particular letter, not with a particular letter.