"Τα βιβλία είναι στη βιβλιοθήκη."

Translation: The books are in the library.

[LukasGroth]

German Bibliothek, French bibliothèque, Italian/Spanish/Portuguese biblioteca, Polish biblioteka, Russian библиотека, ...

Libraries are a Greek thing in many languages. ;)

[em7ec]

How would one distinguish a library and a bookshelf?

[enamrouy]

Or book case?

[AndyPyrope]

They mean a slightly different thing but they are used almost interchangeably nowadays, at least according to what I've read.

[AnCatDubh]

Yeah but dónde está la biblioteca?

[GypsyBarefeet]

It's ok to include the 'ν' in στην? .. . στην βιβλιοθήκη

[G.Georgopoulos]

According to the official grammar, it's not, but virtually nobody pays attention, so, either is ok.

[GypsyBarefeet]

I thought I read that the 'ν' is dropped in informal speech, but kept in formal writing. And, according to newer grammar rules it should be kept.

[G.Georgopoulos]

It is dropped in both formal and informal speech, unless one is speaking reeeeally slowly. The newer grammar rules concern the masculine definite article only.