The Man in Lower Ten by Mary Roberts Rinehart

Amazon | Project Gutenberg
A lawyer traveling by train with important documents is forced by circumstances to sleep in the wrong berth—and wakes to find the documents gone, the man in the other berth murdered, and himself a suspect!

My review of this book.

That Affair Next Door by Anna Katharine Green

Amazon | Project Gutenberg
In this Victorian-era mystery by an author known as the "Mother of Detective Fiction," inquisitive Miss Amelia Butterworth, a forerunner of other lady detectives of literature such as Miss Marple, takes it upon herself to investigate a murder after observing strange goings-on at the house next door.

Lady Audley's Secret by Mary Elizabeth Braddon

Amazon | Project Gutenberg
A nobleman's nephew begins to entertain unsettling suspicions of his uncle's lovely young second wife after the mysterious disappearance of a man who was on his way to see her. Who is Lady Audley, really, and what is she hiding? An entertaining Victorian suspense novel with an engrossing trail of clues to follow.

The Mystery of the Yellow Room by Gaston Leroux

Project Gutenberg | Kindle
An ambitious young reporter and a famous police detective attempt to solve a mysterious murder attempt on a woman in what is regarded as one of the first "locked-room" mystery novels, from the author of The Phantom of the Opera.

(Also available in the original French.)

The Old Man in the Corner by Emmuska Orczy

Amazon | Project Gutenberg
In this collection of short mystery stories, an old man sits in the corner of a café, tying knots in a bit of string, and discusses baffling crimes with a lady journalist, offering his own suggestions about the true solutions.

The Woman in White by Wilkie Collins

Amazon | Project Gutenberg
There, in the middle of the broad bright high-road—there, as if it had that moment sprung out of the earth or dropped from the heaven—stood the figure of a solitary Woman, dressed from head to foot in white garments...
A suspenseful, melodramatic Victorian page-turner, narrated in turn by a succession of different characters caught up in a sinister plot surrounding the mysterious, troubled woman in white. Once started, you won't be able to put it down!

The Circular Staircase by Mary Roberts Rinehart

Amazon | Project Gutenberg

This is the story of how a middle-aged spinster lost her mind, deserted her domestic gods in the city, took a furnished house for the summer out of town, and found herself involved in one of those mysterious crimes that keep our newspapers and detective agencies happy and prosperous.

And with that begins a mystery that falls into the entertaining "mysterious house" category. Noises in the night, guests and servants who disappear, reappear, and generally behave oddly...it's all there. One of Rinehart's first novels, likely to entertain lovers of old-fashioned mysteries.

The Hound of the Baskervilles by Arthur Conan Doyle

Project Gutenberg
Perhaps the best of Conan Doyle's novels featuring Sherlock Holmes, The Hound of the Baskervilles finds the famed detective investigating the mysterious death of a baronet at his Dartmoor estate, tied to a family legend involving a ghostly hound—and a very real danger that may threaten the new heir.

A free edition is not currently available directly for Kindle.

The Red House Mystery by A.A. Milne

Amazon | Project Gutenberg
...But why had he expected the door to be shut. He did not remember shutting it, but somehow he he was surprised to see it open now, to see Cayley through the doorway, just coming into the room. Something working subconsciously in his brain had told him that it was surprising. Why?

Humorist and children's author A.A. Milne only wrote one mystery novel, a sprightly, witty country-house murder mystery that leaves one rather wishing he'd written more. The owner of the Red House is found dead in a locked room, and his disreputable brother, who had been seen to enter the room with him, has disappeared—leaving a pair of friends to play amateur detective and figure out just what happened and how.

My review of this book.

The Innocence of Father Brown by G.K. Chesterton

Amazon | Project Gutenberg
The little priest was so much the essence of those Eastern flats; he had a face as round and dull as a Norfolk dumpling; he had eyes as empty as the North Sea; he had several brown paper parcels, which he was quite incapable of collecting...He had a large, shabby umbrella, which constantly fell on the floor. He did not seem to know which was the right end of his return ticket.
The first of four short-story collections featuring Chesterton's famous amateur detective Father Brown, the mild-mannered, insignificant-looking little English priest with a gift for recognizing the simple solutions to seemingly fantastic crimes.

The Leavenworth Case by Anna Katharine Green

Amazon | Project Gutenberg
Anna Katharine Green, sometimes referred to as the 'Mother of Detective Fiction,' was an American author whose career extended from the 1870s till shortly before World War I. Her books are intricately plotted with complicated webs of clues, and full of Victorian atmosphere and melodrama. The Leavenworth Case, published in 1878, was her first novel and an excellent example of her work.

Uncle Abner, Master of Mysteries by Melville Davisson Post

Internet Archive

"It would be the beginning of justice," said Abner, "if every man followed the standard that God gives him."

In rural Virginia of the early 1800s, Uncle Abner, a stalwart, God-fearing landowner, applies his wits to solving a variety of unusual crimes, in pursuit of justice for both the guilty and the innocent. Told through the eyes of Abner's young nephew, these short stories are fascinating mysteries, beautifully written.

The only place that Uncle Abner is currently available for free is Internet Archive. Books here are usually converted from scans, so often contain formatting glitches and typos, but this copy I have linked to appears to be clean enough to read without trouble. (A 99¢ edition is available for Kindle.)

My review of this book.

The Mysterious Affair at Styles by Agatha Christie

Project Gutenberg

Agatha Christie's very first mystery novel introduced the character of her most famous detective, the round-headed, precise little Belgian Hercule Poirot, with a plot in what would become her classic style: A weathly elderly lady is poisoned at her country house full of relatives and guests, many of whom had motives for wishing her out of the way, and Poirot must determine the culprit in time to save an innocent man.

A free edition is not currently available for Kindle. Note that the Project Gutenberg edition does contain the illustrations helpful to the plot: a diagram of a house and a scrap of handwriting (if you select the "with images" version).

The Adventures of Sherlock Holmes by Arthur Conan Doyle

Project Gutenberg

These twelve classic short stories, including "A Scandal in Bohemia," "The Red-Headed League," "The Man With the Twisted Lip" and more, were the first Sherlock Holmes stories I ever read, and they're an excellent introduction to Sir Arthur Conan Doyle's famed detective, his methods of mystery-solving and his faithful sidekick and chronicler Dr. Watson.

The free Kindle edition is marked as currently unavailable, for unknown reasons.