The Bobbsey Twins, twelve year old fraternal twins, Nan and Bert, and six year old fraternal twins Flossie and Freddie, are the main characters of these beloved books.  This series was one of the longest running ever from 1904-1979 written under the collective pseudonym Laura Lee Hope for several different authors. 

When the series was revised in the 1960s, some of the titles changed ever so slightly and in some cases a little bit more.  You'll want to check one of several sites that lists the books in order, publishers and copyright dates to make sure you have the book you think you have.  

It is a Stratemeyer Syndicate series, which were series of books for girls and boys written from 1905-1985. Their authors wrote over 1400 juvenile series books, like  The Bobbsey Twins, The Hardy Boys, and Nancy Drew, just to name a few.  The man behind all of the books was Edward Stratemeyer, who was an author who started writing in high school and the founder of the this syndicate.  He determined he couldn’t possibly get all of his own ideas and others that he chose for the different series printed under this Syndicate’s name, so he hired various authors who he thought fit the book best to write it.