"Plectrum guitar" can refer to a long necked 4-string guitar tuned like a plectrum banjo. However, in this case, "plectrum guitar" refers to the style of a chord-melody pieces played on steel string guitars (often archtops) using a pick and not fingers. While some many of these were "jazzy", many of them weren't really jazz. Essentially they were like classical guitar pieces, but updated for the harmonic vocabulary (i.e. jazz), and equipment (i.e. steel string guitars and picks) of the day. The Mel Bay Book, which I've mentioned before on here, "Masters of the Plectrum Guitar" uses that terminology and is a vital collection of Lang/Kress/McDonough and other similar guitar pieces.
But another of the best resources of this style is the website of Scottish guitarist Rob MacKillop.
Rob is someone I've come across mostly on the Just Jazz Guitar Board (they still have online forums?! no way!), and he's always been a nice guy and an advocate for the acoustic archtop as instrument worthy of consideration separate from the electric archtop.
I was browsing the forum a couple months ago, and noticed a video he'd posted of a couple Roy Smeck solo guitar tunes from 1928. This caught my eye because I'd just been working on learning pieces out of the Allan Reuss book, I was primed to explore some of the early "plectrum" guitar pieces of folks like Smeck, Nick Lucas, Eddie Lang, Frank Victor and Harry Volpe.
Once I clicked through to Rob's website, archtopguitar.net, I was blown away by there being a lot more than just the two Smeck pieces. The blog had posts the included not just videos of Rob expertly playing many, many pieces from this mostly ignored repertoire, but also posting the sheet music.
For my part, I jumped into learning "Itching Fingers" by Roy Smeck. And it inspired me to go back into the Mel Bay book above, and learning "Pick It and Play It" by Frank Victor. Between those tunes and two Reuss tunes I've learned, that's four vintage plectrum guitar pieces I've placed into my solo guitar repertoire, along with my own chord-melody arrangements.
Take a look at Rob's site, and consider donating as well.