I was noodling around with the Play Index and did a search for batters who hit 20 or more HRs in their first season in the bigs, but restricted it to players who debuted at age 30 or older. I figured there couldn't be too many since anyone capable of hitting 20 HRs at the highest level of competition would probably have managed to get to the majors before age 30 or would have had other problems of such significance that they would never get there.
Turns out there aren't any.
So I lowered the minimum HR total to 15 and came up with only 6. Kenji Johjima tops the list with 18 and Tadahito Iguchi finishes it with 15. They're from Japan and played professionally there before coming here, so it's not too surprising that they could make this list.
The second listing, Sam Jethroe, also had 18 HRs and he was a player in the Negro Leagues (I should have realised that the colour barrier might come into play for people on this list) for several years and was given a tryout by the Red Sox in 1945, though according to this link (usual caveats about not knowing anything about the rest of the site) this was a rigged tryout that was never intended to offer any of the black players a real chance at joining the team. He only played three seasons (his fourth year he only appeared in 2 games and had merely 1 AB) while putting up decent numbers from the plate, but numbers in the outfield that are worse than Manny Ramirez. He led the league in steals for a couple years and when his offensive production dropped he went to the minors.
The third man on that list with 18 HRs is Buzz Arlett. A very interesting player who set records for minor league play and had an excellent year batting in his sole year in the majors. Most of the searching I did online indicated that it was a lack of fielding prowess that kept him from playing at the highest level for more than a year. Indeed, he had a high number of errors relative to his number of fielding chances. Someone who would have benefited a great deal from the introduction of the DH, I would guess.
I feel sorry for Jim Baxes, number four with 17 HRs. He played but a single year, starting with and for the Dodgers who traded him after 11 games despite his good numbers to that point. The man for whom he was traded never played a day for the Dodgers and LA went on to win the World Series that year. The man who replaced him at third had just as many errors and hit for less power though a better average.
The fifth on the list, George Watkins, had a longer career than any of the rest (excluding the Japanese players). He played seven full seasons and his first year hit a stellar .373, which was good enough for...sixth place on the batting title list?! Yep, he had the misfortune to come up the year Bill Terry hit .401 and another four players all hit .379 or better.