Each clue is a Latin phrase as it might be interpreted by someone who doesn't really know any Latin.
|One who doesn't say "thank you"
|Rental company that's hard to find
|Wagon that attracts things
|Lament from "On the Waterfront"
|Poem about one's foibles
|Warning when you're almost out of gas
|Why everyone hears when spring ends
|Between SDTV and HDTV
|Why thinkers become accountants
|Italian gaming convention
|Druopta Efekta, e.g.
|Where the cause of some teenage grief lies
|Former place of worship
|Crucial course material
|Like fans of U2 concerts
|A really, really long commercial break
|Two, three, or five on a die
|Opposite of a lax tenon
|How one sits
|Like an unfinished clock tower
|Al Qaeda's legal team, e.g.
|Result of starting a sit-up regimen
|What non-amateurs get paid
|Solution to: x2 + cos2(0) = 1
|To pursue a non-specific lawsuit
|The result of further radiological dumping
|"The Pony Express began in April 1860," e.g.
The indicated blanks in order spell NICODEMUS GOSPEL ON CORE FITNESS. The Gospel of Nicodemus does not contain anything about the Pilates fitness system, but it does contain a text known as the ACTA PILATI, which is the answer.