How/when does one use "a most"?

I've recently come across a novel called A most wanted man, after which being curious I found a TV episode called A most unusual camera.

Could someone shed some light on how to use "a most" and whether it has anything to do with "the most"?

"Most" in this context means "very." As in "a very wanted man." It functions as a "superlative."

This usage is considered "dated" and is not recommended for modern contexts. I am providing this explanation for people who want to read (or possibly write) earlier "period" novels or shows.

This is a special kind of absolute superlative. The OED gives as its sense 2 of most adv. the following:

As an intensive superlative qualifying adjs. and advs.: In the greatest possible degree.
_{For the ceremonial titles, Most Christian, Most Honourable, Most Noble, Most Reverend, etc., see those adjs.}

I don’t think it gets used all that very much anymore. One of the examples is:

1710 Hearne Collect. (O.H.S.) II. 351 ― A most vile, stinking Whigg.