Given the era of the boys' series books contained in e-book archives, it was a given that the boys would experience more adventure and danger than their female counterparts.
Boys were definitely boys in those days, and while the girls did get to solve mysteries, frequently face unpleasant people, and even encounter spies, the boys invented, fought in wars or against enemies, were shot at, participated in long trail rides or motor rides, hunted and faced wild animals (not to mention wild men—outlaws and such), occasionally used their fists, always used their wiles—and remained gentlemen to boot!
The granddaddy of all boys series' was the Rover Boys. They're quaint reading now, even for 19th century book fans: they speak in pseudo-British slang (the opening stories take place in the boys' boarding school, Putnam Hall, and their schoolmates were spun off into yet another series). During their adventures the boys age, eventually marry, and their sons take over the youthful tales.
Probably the most famous of the boys' series was Tom Swift. Despite decades of "tom swifties" jokes, the Tom Swift series was a serious set of books about young Tom, a budding inventor and his inventor father, and what seemed like the constant efforts of other, lesser inventors to steal Mr. Swift's blueprints, patents, etc. The first books discussed machinery like motorcycles and motorboats, which, while commonplace to us, were quite novel to the readers of 1910. Later inventions included electric guns and other more fanciful equipment. Tom, like the Rover Boys, aged, married, and had a son, Tom Swift Jr, who participated in more space-age adventures with rocket ships and ray guns.
As time passed, the boys' series leads were often paired with the newest technology. There were boy motorcyclists, boy radio operators, boy motorboat owners, boy motorists, boy submariners, and, of course, boys involved with the newest, most amazing invention of all, the airplane. There were several boy aviator series, most of which took place during "the Great War." Their names changed, but the plots mostly followed the same plots: the gallant American boys flying for freedom against the "Boche" (Germans), saving the helpless and foiling evil spies. (If you blanch at the thought of ten-year-olds flying airplanes, driving cars and motorcycles, and thwarting enemies, please recall that the term "teenager" had not been coined when these books were written. The "boys" represented in series books were usually seventeen to nineteen years old.)
A more typical set of boys to begin with were H. Irving Hancock's "High School Boys," initially a set of sports-themed books. "Dick & Co.," as they were often known, after their leader, Dick Prescott, also included Greg Holmes, Dave Darrin, Dan Dalzell (also known as "Danny Grin"), Tom Reade, and Harry Hazelton, all of whom lived in the town of Gridley and attended Gridley High. Well-mannered, honest "square fellows," they were nevertheless frequently tormented by the town bully and his toadies. Hancock followed with a summer series about the boys, and then, having exhausted their later schooling, wrote a series of "Grammar School Boys" adventures about them. Once the boys reached adulthood, however, they split into three groups, each with a series. Dick and Greg went to West Point, then became "Uncle Sam's Boys" and fought in Europe. (Dick, amazing as always, on his first day in the trenches spots a spy who has fooled everyone else, including the officers, for weeks!) Dave and Dan went to Annapolis, then participated in various famous naval encounters, including adventures in Vera Cruz. Very soon our young lads were officers commanding small ships! Tom and Harry took a less military approach and attended engineering school, then managed to be coupled on assignments in various wild places, including Mexico and Nevada, where they always earned the lasting hatred of the biggest, baddest gunslinger around (who, of course, they always bested with superior thinking).
One of the more unusual series was the five-book "Circus Boys" series. Phil and Teddy, two bored country boys, join...well, the circus, starting out as general dogsbodies and tenderfoot performers, and by the end of the series, due to their pluck, initiative, and staying power, have worked themselves up to be publicists for the show, before even hitting their 20s. It was an interesting look behind the workings of an early 1900s traveling circus.
Again, one cannot discuss these early series, boys' or girls', without mentioning the sometimes blatant, sometimes more subtle, racism that was included in the stories. One of the most memorable characters in the original Tom Swift series, for example, is Eradicate Andrew Jackson Abraham Lincoln Sampson, the typical minstrel-show African-American supporting character. While "Rad," as Tom calls him, often provides Tom with important "clews" and saves him, he also speaks in stereotypical "colored dialect" and prefers to travel with his mule, Boomerang. These sad individuals spot both boys' and girls' series, for both younger and older children: Dinah and Sam in the Bobbsey Twins and Hercules, the family retainer in two of the Frey Camp Fire Girls books (another stereotypical African-American provides a plot point in yet a third Frey book) are only two of many examples, which is why I was surprised to discover The Air Ship Boys by Saylor, which contained a young black character who spoke in dialect, but not as bad as other novels, and did not act the fool; while he did most of the scut work for his white companions, he was also considered a trusted guard and companion.
Most minorities and many ethnic groups came in for drubbing. The worst series in this respect was the Pony Rider Boys series, about a group of boys and their older escort, who visited various Western and wilderness locales on horseback, encountering rustlers, cheats, land grabbers, and other villainous types. I had a tough time making it through my first Pony Rider book, despite thrilling adventures, faced with such epithets as "greaser" and "spic" directed to every Mexican, "dirty savages" in regards to Native Americans, "Chinamen" to Asians, etc. As observed in my girls' series entry, it is not surprising white children who read these books came away with such racist attitudes, as these books were provided to them by people they respected: parents, relatives, friends, perhaps even clergy.
Again, however, I still maintain these books have much to offer. They portray the virtues the early 1900s' adult wished good, manly boys to have: honesty, courage, conviction, drive, steadiness; open a window to the technological advances of the time; portray the closing frontiers of the United States and even Europe; show how the First World War was "spun" as a great crusade even to the youngest citizens of the United States; and vividly point out what strides have been made in defeating cruel and ugly stereotypes. It is a trip back in time that displays the realities, good and bad, of the era in a much more honest way than more recently written books that try to pretend that such things were not common among educated persons.