Red Storm Rising by Tom Clancy (and Larry Bond) was published in 1986 and instantly became a best-seller. The novel told the story of a war in the near future (signs point to 1987) between NATO and the Warsaw Pact. This topic had been covered previously in fiction, but Clancy brought an an attention to particular technologies as well as characterization that earlier efforts such as The Third World War: 1985 by retired British general Sir John Hackett and friends had not quite managed.

Among the technological wonder weapons featured in the novel was the F-19A Ghostrider stealth fighter. Nicknamed the "Frisbee" by its crews because of its radar-defying curved shape, the Clancy's F-19 helped NATO launch a devastating aerial counterattack on the first night of the war that set the stage for NATO's defensive effort, which prevented the Pact from winning a quick victory. One of the novel's minor characters, Col. Douglas "Duke" Ellington, is a USAF F-19 pilot and squadron leader, who along with backseater Maj. Don Eisly flies some of the toughest missions.

You are right about Popular Mechanics. In fact, for about a decade before the 1988 disclosure of the F-117A Nighthawk (the actual USAF stealth fighter) the idea of a stealth fighter was already in the consciousness of aviation and defense enthusiasts. A little bit was of information was known to the interested public about the USAF's actual stealth fighter program, including the fact that famed Lockheed designer Kelly Johnson was involved. Based on this information and a perceived market, earlier in 1986 two hobby companies (Testors and Monogram) had released plastic kits of an "F-19 Stealth Fighter." They sold extremely well. (Testors, whose model was based on some pretty good detective work, followed up the next year with a conjectural MiG-37 Ferret, the Soviet stealth fighter.) In addition, the stealth fighter became a top news story when a real one crashed in California on July 11, 1986, and the efforts to secure the crash site attracted attention. So the existence of a USAF stealth fighter was, by 1986, an open secret, though the public didn't quite know what it looked like.

The F-19 designation, by the way, comes from the fact that the F-for-fighter series that had been commenced in 1962 skipped F-19. The F-14 Tomcat, F-15 Eagle, F-16 Fighting Falcon, and F-18 Hornet were the armed forces' hottest new fighters. The F-17 had lost a competition to the F-16 (but served as the basis for the F-18). The F-20 Tigershark was a Northrop product designed for the export market that ultimately flopped. When the Navy leased some Kfir fighters from Israel for adversary training (basically, playing the bad guys in Top Gun-like programs), they were designated F-21. What happened to F-19? It had to be the missing stealth fighter! It turns out the Department of Defense had skipped the number on request from Northrop, who thought the "F-20" designation would make their product stand out. The actual F-117 designation was completely unguessable because the process for determining it was highly idiosyncratic.

Comparing Clancy's Ghostrider to other ca. 1986 ideas about a stealth fighter as well as the actual F-117 shows that Clancy's is probably the most capable. Clancy's Ghostrider is supersonic and capable of air-to-air combat as well as ground attack. It carries a substantial external ordnance load. It seems to be a somewhat large aircraft since it has a crew of two. One way to think about the Ghostrider is that it's basically an F-15E Strike Eagle (two-seat long-range strike aircraft with air-to-air capability) that is mostly invisible to radar. By contrast, the Testors F-19 model was a single-seat aircraft optimized for ground attack, carried its weapons internally, and was described as subsonic--which was actually public information at the time because it was known the stealth fighter used a non-afterburning engine. The actual F-117 Nighthawk was quite similar to the Testors model except for its precise shape. The F-117 is highly angular whereas many of the conjectural stealth fighters were seen as curved; Clancy's F-117 is nicknamed "Frisbee" for this reason. In fact, Johnson had proposed a curved design and those who knew he was on the project suspected he would come up with a curved design, but an angular design proved better for reducing radar cross-section. The angular rather than curved design was the biggest surprise when the F-117 was revealed in 1988. Clancy's decision to let his stealth fighter carry weapons externally is completely wrong, since this greatly increases radar cross section.

So, to sum up, by the time Clancy was writing Red Storm Rising, the idea of a stealth fighter designated "F-19" was familiar to the novel's target audience. His version of the stealth fighter was actually not the most accurate of those in circulation and included several capabilities and design decisions that aren't in the F-117 and probably would not have worked.

This 1986 article on the Testors model gives you a good look at what was figured out from open sources about the stealth fighter: https://www.latimes.com/archives/la-xpm-1986-10-19-tm-5852-story.html

Are you certain of the book title?

There's certainly a Red Storm Rising by Clancy, published in 1986. But as far as I know, Clancy's debut novel was "The hunt for Red October" in 1984.

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I understand that future fighter jets were publically referenced by Loral Corporation in 1984, in Aviation Weekly (I believe that this was published in the January 23rd, 1984 edition, but I don't have subscription to the Aviation Weekly archives to confirm that), using a concept painting produced by Attila Hejja (1954 - 2007), a Hungarian-born, New York-based concept artist, which was produced for the DoD in late 1981 or 1982.

Here's the concept art in the US national archive:
https://catalog.archives.gov/id/6343685

so this was certainly in circulation in 1982, likely before such a book was written - the same artwork became used as the inspiration for a series of model kits using the F-19 name in the mid-1980's by companies like Testors, and Revell, which were popular at the time. (excuse anecdotal evidence, but I certainly remember reading speculation about such an aircraft, based on those model kits in in the mid-80's, in "National Geographic" magazines. )

So it would be reasonable to assume that an author like Clancy who was keeping abreast of recent technological hardware would've found the artwork through popular publications like "Aviation Weekly" and the likes.