Brackets around a single letter? For example. [L]ibraries are a source of knowledge?

That's because in the original text, that word had a different capitalization then in the quoted text. For example in the original text it was the first letter of a sentence and had to be capitalized, but when it is quoted they change the capitalization to fit its place in the new sentence. If I quote someone saying:

Cats are very good mouse catchers.

I would say that experts have said that "[c]ats are very good mouse catchers."