There are actually a very large number of free Harper titles for Australians. I've listed the ones I've found below, some of which are short stories, but many appear to be full novels. The easiest way to see them is from this search. I doubt any of these will end up free in the US, but anything is possible.
- Halfway to the Grave with Bonus Material: A Night Huntress Novel by Jeaniene Frost
- Happily Never After by Jeaniene Frost
- A Very Vampy Christmas: From Sugarplums and Scandal by Kerrelyn Sparks
- Rose in Bloom: A Novella with Bonus Excerpts by Stephanie Laurens
- Fatal Heat: A Navy SEAL Novella by Lisa Marie Rice
- Royal Weddings: An Original Anthology by Stephanie Laurens, Gaelen Foley and Loretta Chase
- Reckoning: From Unbound by Jeaniene Frost
- A Lady's Wish by Katharine Ashe
- The Amanda Project: Book 2: Revealed: Part 1: Chapters 1-3 by Amanda Valentino and Peter Silsbee
- Diaries of an Urban Panther by Amanda Arista
- Before the Witches: An Original Novella by Karina Cooper
- He Kills Me, He Kills Me Not by Lena Diaz
- Dark Future by Kc Klein
- The Perfect Hero by Victoria Connelly
- The Treats Truck Baking Book by Kim Ima
- 1066: History in an Hour by Kaye Jones
- American Slavery: History in an Hour by Kat Smutz
- Ancient Egypt: History in an Hour by Anthony Holmes
- Henry VIII's Wives: History in an Hour by Julie Wheeler
- Hitler: History in an Hour by Rupert Colley
- Nazi Germany: History in an Hour by Rupert Colley
- The Reformation: History in an Hour by Edward A Gosselin
- World War Two: History in an Hour by Rupert Colley
- Les Très Riches Heures de Mrs Mole by Ronald Searle
- Sawbones: A Novella by Stuart MacBride
- Clues to Christie: The Definitive Guide to Miss Marple, Hercule Poirot and all of Agatha Christie's Mysteries
- Sharpe's Company, Sharpe's Sword, Sharpe's Enemy by Bernard Cornwell
- Frankenstein Special Edition: Prodigal Son and City of Night [ePub edition] by Dean Koontz
- 77 Shadow Street by Dean Koontz
- Brought in Dead (Nick Miller 2) by Jack Higgins
- Sam Bourne Bundle by Sam Bourne
- Sheba by Jack Higgins
- The Great Bazaar and Brayan's Gold by Peter V. Brett
- Michael Stanley Bundle: A Carrion Death & The 2nd Death of Goodluck Tinubu: The Detective Kubu Mysteries with Exclusive Excerpt of Death of the Mantis by Michael Stanley
- JENNY LOPEZ HAS A BAD WEEK by Lindsey Kelk
- Twelve Days of Winter: Crime at Christmas (3) - French Hens by Stuart MacBride
- Twelve Days of Winter: Crime at Christmas (4) - Calling Birds by Stuart MacBride
- Twelve Days of Winter: Crime at Christmas (7) - Swans A Swimming by Stuart MacBride
- Twelve Days of Winter: Crime at Christmas (9) - Ladies Dancing by Stuart MacBride
- Twelve Days of Winter: Crime at Christmas by Stuart MacBride
- Twelve Days of Winter: Crime at Christmas (1) - A Partridge in a Pear Tree by Stuart MacBride
- Twelve Days of Winter: Crime at Christmas (2) - Turtle Doves by Stuart MacBride
- Twelve Days of Winter: Crime at Christmas (5) - Gold Rings by Stuart MacBride
- Twelve Days of Winter: Crime at Christmas (10) - Lords A Leaping by Stuart MacBride
- Twelve Days of Winter: Crime at Christmas (6) - Geese A Laying by Stuart MacBride
- Twelve Days of Winter: Crime at Christmas (8) - Maids A Milking by Stuart MacBride
- Twelve Days of Winter: Crime at Christmas (11) - Pipers Piping by Stuart MacBride
- Twelve Days of Winter: Crime at Christmas (12) - Drummers Drumming by Stuart MacBride
- Warriors Digital Novella by Erin Hunter
- Hunting Kat by Kelley Armstrong
- Trafficked: The Terrifying True Story of a British Girl Forced into the Sex Trade by Sophie Hayes
- Birthdays for the Dead by Stuart MacBride
- The Dice Man by Luke Rhinehart
- Grumpy Old Men: New Year, Same Old Crap by David Quantick
- The Time of My Life by Cecelia Ahern
- Pitch Dark: Dark Days of Fall Sampler: Supernaturally; Fateful; Cold Kiss; A Beautiful Dark; and Eve by Amy Garvey, Claudia Gray, Anna Carey and Kiersten White
- Bleak Water by Danuta Reah
- Wishing for Snow by Minrose Gwin
- PitchDark Presents the Dark Days of Supernatural Paranormal Romance Sampler by Various
- The Loving Daylights (B.L.I.S.S.) by Lynsay Sands
- Beyond Moralism by John Shelby Spong
- American Hunger by Richard Wright
- The Easter Moment by John Shelby Spong
- Parting Gifts by Lorraine Heath
- Sweet Lullaby by Lorraine Heath
- Texas Destiny (Leigh Family) by Lorraine Heath
- Texas Glory (Leigh Family) by Lorraine Heath
- Every Year: Short Stories by Cecelia Ahern
- A Taste of Irrationality: Sample chapters from Predictably Irrational and Upside of Irrationality by Dan Ariely
- A Girl's Guide to Vampires (Dark Ones) by Katie MacAlister
- Stopping Time (Wicked Lovely) by Melissa Marr
- Stepping Out Within by Robert W. Olsen
- After Dark (Dark Realm) by Delilah Devlin
- Another Door by Jess Michaels
- Fallen Angel by Jess Michaels
- It's in His Kiss: The Epilogue II (Bridgerton Family) by Julia Quinn
- Lady Amelia's Secret Lover by Victoria Alexander
- The Lover Hunt by Jess Michaels
- Magic and Mayhem by Sylvia Day and Cathryn Fox
- Parlor Games by Leda Swann
- Polly's Game by Leda Swann
- Que Se Yo del Universo (Don't Know Much About...) (Spanish Edition) by Kenneth C. Davis
- Unrequited by Kimberly Dean
- Victim of Deception by Lynn Lafleur
- Agent Provocateur by Liz Maverick
- If the Holy Spirit Comes for You by Barb Johnson
- Issue Is by Barb Johnson
- Killer Heart by Barb Johnson
- Titty Baby by Barb Johnson
- What Was Left by Barb Johnson
- Allegory of a Cave by Holly Goddard Jones
- Good Girl by Holly Goddard Jones
- Life Expectancy by Holly Goddard Jones
- An Offer From a Gentleman: The Epilogue II (Bridgerton Family) by Julia Quinn
- Parts by Holly Goddard Jones
- Proof of God by Holly Goddard Jones
- Theory of Realty by Holly Goddard Jones
- To Sir Phillip, With Love: The Epilogue II (Bridgerton Family) by Julia Quinn
- Mistress of Paradise by Alexandra Benedict
- The City of Windy Trees (P.S.) by Simon Van Booy
- The Coming and Going of Strangers (P.S.) by Simon Van Booy
- Love Begins in Winter (P.S.) by Simon Van Booy
- The Missing Statues (P.S.) by Simon Van Booy
- Imponderables (Imponderables®) by David Feldman and Kas Schwan
- Reunion by Lynn Lafleur
- Romancing Mister Bridgerton: The Epilogue II (Bridgerton Family) by Julia Quinn
- These Wicked Games by Sherry Ledington, Courtney Milan, Lacey Kumanchik and Eve Ortega
- The Viscount Who Loved Me: The Epilogue II (Bridgerton Family) by Julia Quinn
- When He Was Wicked: The Epilogue II (Bridgerton Family) by Julia Quinn
- Escape the Diet Trap by Dr John Briffa
- The Hunting of Man: A History of the Sniper by Andy Dougan
- Collins Know Your World Series - China
- Collins Know Your World Series - Egypt
- Collins Know Your World Series - Germany
- Collins Know Your World Series - India
- Collins Know Your World Series - Japan
- Collins Know Your World Series - Nigeria
- Collins Know Your World Series - UK
- Guys Read: Thad, the Ghost, and Me by Margaret Peterson Haddix
- Guys Read: The Snake Mafia by Gennifer Choldenko
- Guys Read: Believing in Brooklyn by Matt De La Pena
- Guys Read: "What? You Think You Got It Rough?": A Short Story from Guys Read: Funny Business by Christopher Paul Curtis and Jon Scieszka
- Casper Candlewacks in the Claws of Crime! by Ivan Brett