"The books are in the library."
Translation:Τα βιβλία είναι στη βιβλιοθήκη.
I saw that it has been added, but I'd like to add that -maybe it's a matter of personal taste, don't know- "μέσα" is redundant and infrequent when one's talking about big, enclosed spaces, because the books are probably never going to be on, next to, under or above a library.
I'm pretty sure I read in a different thread, that in cases where -ν is not necessary, omitting the -ν is optional, while in the cases where -ν is necessary, it is obligatory to have it. In other words, as I understood it that both 'στη/στην βιλβλιοθήκη' would be correct while 'στη πέτσετα' would be wrong. Or does this only apply to the masculine στον/στο situation?
Exactly. The nu is retained before plosives and affricates, and β is no longer a plosive -- nor are δ, γ, φ, θ, χ, all of which became fricatives centuries ago.
Today's plosives are π τ κ μπ ντ γκ /p t k b d g/ and the affricates (that start with a plosive) are ψ τσ ξ τζ /ps ts ks dz/.
Oddly enough, there's only one voiced affricate; there aren't any /gz/ or /bz/ sounds, just /dz/. (Perhaps because of the source of the sound: I think /dz/ is mostly if not solely found in loanwords, especially from Turkish.)