The first 19 books were written by W. Bert Foster. Since the first 19 books were all written by one person, there are no noticeable lapses in consistency.
What is most striking once the books are written by Ward and Wirt is that subtle changes occur in the characterizations and in the interaction between the characters. It's not something that I can easily describe here, but the way Helen Cameron acts and the way that Helen and Ruth interact definitely changes once the books are no longer written by Foster. I feel like the relationship between Ruth and Helen is more interesting for the reader during the books written by Ward, and I am not far enough into the books written by Wirt to have an opinion.
Volumes 20 through 22 were written by Elizabeth M. Duffield Ward. Except for what I have already stated above, these three stories are pretty consistent with the first 19 books. There is one thing that stands out. Ruth is suddenly disaster-prone, just like the ever unfortunate Blythe Girls. This is amusing, since Elizabeth M. Duffield Ward wrote the entire Blythe Girls series under the pseudonym of Laura Lee Hope.
Ruth Fielding in the Far North is the best example of the Blythe Girls phenomenon and is my pick for the most exciting book (of those that I have thus far read) in the Ruth Fielding series. Ruth's friends Chess and Helen have a serious car accident on page 11. On page 27, the boat upon which the friends are to sail is set afire. After repairs, Ruth and her friends depart. On page 39, the boat hits an iceberg, which rips a hole in the boat. The boat is patched, and the journey proceeds. On page 46, Helen falls overboard into the icy water. On page 58, the boat is caught in the ice, wedged in tightly where it cannot move. On page 65, the young people abandon the boat and set off across the "rubber ice" in a risky attempt to make it to land. On page 88, Ruth shoots at a polar bear that is about to attack. On page 120, the young people's boat is charged by an enraged bull walrus. On page 135, Ruth is lost in the snow, and by page 139, Ruth stumbles into a deep pit and is trapped with two bears. On page 159, Tom is knocked unconscious and falls into the icy water. Ruth plunges in to save him, and both nearly lose their lives. On page 185, the young people are lost in a blizzard and seek refuge in an abandoned igloo.
The other two Ruth Fielding books written by Elizabeth M. Duffield Ward also have more exciting events than the earlier Ruth Fielding books, but not as extreme as the ones I have mentioned.
I just finished reading volume 23, Ruth Fielding and Her Great Scenario, which is the first Ruth Fielding book that was written by Mildred Wirt Benson. In volume 23, Ruth's main interest is in working on her scenario for a contest, but in the latter part of the book, Ruth and her friends do a little detective work in regard to portions of her scenario that have been stolen. Now the detective work is not odd, since it is strongly tied into what is most important to Ruth—her career. The detective work is important because it is the first indication of the shift that occurs during the next book.
In volume 24, Ruth Fielding at Cameron Hall, Ruth loses her money in a bank robbery, and at the same time, Tom Cameron disappears. The two events seem to be related. Ruth vows to solve the mystery herself. From pages 54-55:
Ruth tried to hide her irritation. She understood Si Perkins and his methods. Obviously, if she waited for him to locate the guilty persons, she probably would never again see her forty thousand dollars. And Aunt Alvirah was depending upon her for the recovery of "the egg money!" Uncle Jabez Potter would always feel that he had lost his savings because he had followed her advice and had placed his money in the bank.It was with this passage that I realized that Ruth Fielding is morphing into Nancy Drew! It reads just like one of the passages from any of the early Nancy Drew books. Ruth has the same disdain for law enforcement as Nancy and feels like she can do a better job than they can, just like Nancy.
"I've sent for some city detectives," the sheriff went on, with a touch of pride. "They ought to be here some time to-day."
Ruth resisted a desire to reach out and shake the man. Did he never think for himself? "In the meantime, where will your thieves be? Miles away, of course. They won't wait around for the city detectives! Why don't you start out in cars and search the country around here? Surely, Mr. Jones knows in what direction the roadster was traveling when he saw it start away from the bank."
Ruth turned away. Little chance Si Perkins and his posse would have of running down the gang of criminals in the blue roadster unless they showed more skill and speed than they had previously.
Milded Wirt's first assignment for the Stratemeyer Syndicate was Ruth Fielding and Her Great Scenario. Edward Stratemeyer gave it to her to see what she could do with it and whether she had what he needed in a writer. She submitted the first chapters and had to make a good many changes in order to give him what he wanted, but by the time she was finished with the manuscript, he was quite pleased. Wirt wrote the rest of the Ruth Fielding books and was given the Nancy Drew series as her next assignment. The rest is history.