-thek-? (German)

Here's one I found while reading Thomas Bernhard's Der Stimmenimitator:

Pharmacist - Apotheker

Ok, BUT!

Librarian - Bibliothekar

So why the -ar ending? Is this a holdover from Greek or some strange anomaly? Are there other -thek endings that do this? I can think of one other: Diskothek, but I would imagine that one who frequents Diskos would be a Diskotheker, right?

Also, has anyone read The American Language by H.L. Mencken? Is it generally respected?

(Half-Crossposted in germanics)