Ep. 15: The Northwest gave us one of the best mysteries of the 20th century… the polite parachuting hijacker who’s (erroneously) known as D. B. Cooper. In the first part of this two-parter episode, Devon outlines the facts of the case, the FBI’s approach in the early days of the hunt, and the few clues Cooper left behind.
Secondary topics: A name fake-out; no smoking on planes please; a parachute fake-out; just a general intermittent clean-up search; no sack lunch, no windbreaker; like a seagull with a french fry.
Ep. 17: In the conclusion of our D. B. Cooper episodes (unless some fresh new evidence emerges!), Devon outlines what we know about the case, what we assume about the case, and the cases for and against three plausible suspects.
Secondary topics: Never watch a mystery with Liz; the Federal Bureau of Stuff We Just Really Wanna Know; club moss in druggums; a new, genie-based theory; cases in which you get what you pay for; Looney Tunes hijacking; Devon and Liz cross wires on their eighties references.
Devon read dozens of articles in her quest to find the missing 200,000-ish dollars, but relied most heavily on the following articles and resources:
The man himself *     
Citizen Sleuths 
FBI case file 
Copycats  
*(Wikipedia is a thorough aggregation of the evidence and timeline and was used almost exclusively by Devon when retelling the hijacking)