Hah, well they're sorta like Hardy Boys or Nancy Drew but I think you read them at a slightly younger age (ie I think I was 8 or so when I started reading this series, Nancy Drew I didn't start until I was about 10). And hey if it's on Project Gutenberg they've definitely been around awhile.
I will also say though how I think one of the most fun parts of re-reading stuff you haven't touched in decades is how various lines stick out at you. The following two are from the end of the book when the kids are moving in to live with their wealthy grandfather.
This was the first that his grandfather had heard about school, but he agreed with Henry, and chuckled to himself.
"The finest schools in the country," he said. This came true, for all the children finally went to the public schools, and are they not the finest schools in the country?
"What am I going to do when I grow up, Grandfather?" asked Henry.
"You're going to take my place, Henry, as president of the steel mills," replied Mr. Cordyce. "You will do it better than I ever have." (And one day this came true, just as most of Mr. Cordyce's prophecies did.)
"And what am I going to do?" asked Jess, curiously.
"All you children must go to school and then to college. Then you may do whatever you choose for a living," replied Mr. Cordyce. (This also came true.)
"Of course I have more than enough money to support us all," went on Mr. Cordyce, "but if you have something to do, you will be happier." (This not only came true, but it is always and forever true, all over the world.)
I don't know why, but I somehow find the former an interesting editorial and the latter a lovely truism to put into a children's book.