There are 9 books. At least 5 of them are PD in the US and available at Gutenberg:
- Billie Bradley and Her Inheritance; Or, The Queer Homestead at Cherry Corners https://www.gutenberg.org/ebooks/10048
- Billie Bradley at Three Towers Hall; Or, Leading a Needed Rebellion https://www.gutenberg.org/ebooks/23894
- Billie Bradley on Lighthouse Island; Or, The Mystery of the Wreck https://www.gutenberg.org/ebooks/25762
- Billie Bradley and Her Classmates; Or, The Secret of the Locked Tower https://www.gutenberg.org/ebooks/40586
- Billie Bradley at Twin Lakes; Or, Jolly Schoolgirls Afloat and Ashore Not yet PD?
- Billie Bradley at Treasure Cove; or, The Old Sailor's Secret Not yet PD?
- Billie Bradley at Sun Dial Lodge; or, The Chums Solving a Mystery Not yet PD?
- Billie Bradley and the School Mystery; Or, The Girl From Oklahoma https://www.gutenberg.org/ebooks/50157
- Billie Bradley Winning the Trophy; or, Scoring Against Big Odds Not yet PD?