Captain America, created by Joe Simon and Jack Kirby (unlike most Golden-Silver Age characters, Stan Lee was not involved) in the summer of 1940, was one of the biggest characters of Timely Comics in the 1940s, one of many, many patriotic characters created in response to the environment of World War II. Following the end of the war, he, like most superheros, dropped off the publication radar, despite a brief failed attempted revival in the early 1950s. The latter attempted revival was so embarrasingly McCarthyist in its sentiments that Lee and Kirby would later disavow it when they opted to bring Captain America back in 1964, when the Silver Age of Comics was starting up. Following his return in "Avengers #4", Cap eventually got his own run of solo stories, co-sharing "Tales of Suspense" with Iron Man for a good part of the 1960s. This trade paperback collects those portions of issues 59-81 (so, about twelve issues of material). Some (40-year-old) spoilers follow.
There are a couple of different story strands at work here. After a couple of short adventures set in the modern (1960s) era, we get a lengthy series of adventures set in the World War II era, with Captain America and his sidekick Bucky facing off against the Red Skull, the archvillain of the German armed forces. Steve and Bucky have several daring adventures, all the while posing as ordinary soldiers, with Steve doing the standard Clark Kent schtick as a bumbling private for his frustrated drill sergeant. After this time-trip, the narrative focus returns permanently to the 1960s, as Steve, now an Avenger, strikes off on a few solo adventures of his own, including a new teamup with old ally Nick Fury (now head of SHIELD), and his first meeting with his eventual love interest Agent 13. The most famous story here is likely his first encounter with the Red Skull in the modern era, who wields the infinite power of the Cosmic Cube (which would go on to be a recurrent element in the Captain America mythos).
Silver Age comics are always a bit of an acquired taste for people. These stories were quite revolutionary in their day, but they can't help but be rather dated to modern eyes (not that different from 1960s film and television, one supposes). All the same, there are things to enjoy, and the stiltedness can even be part of the fun, after a while. Consider, for example, the origin of the Red Skull, a bellhop who is drafted to become Hitler's right hand because the Fuhrer can tell, from looking into his eyes, that he shares his hatred for the entire world. The most nuanced depiction of fascism, this is not, but it's enjoyably over the top. The bits with World War II-era Steve pretending to be a bumbling soldier strike one as sort of pointless, but then, this was an era where superheroes always had to have a secret identity, even when it didn't really suit the character. The art, mostly by Jack Kirby, but with contributions by others, including John Romita Sr., is as good as any from the period.
If you like Silver Age comics and want to see some of the early adventures of one of Marvel's classic characters, give this a look.