Some out of copyright (aka free) I assume would suit:
Murder in the Gunroom by H. Beam Piper. Synopsis: "The Lane Fleming collection of early pistols and revolvers was one of the best in the country. When Fleming was found dead on the floor of his locked gunroom, a Confederate-made Colt-type percussion .36 revolver in his hand, the coroner's verdict was "death by accident." But Gladys Fleming had her doubts. Enough at any rate to engage Colonel Jefferson Davis Rand--better known just as Jeff--private detective and a pistol-collector himself, to catalogue, appraise, and negotiate the sale of her late husband's collection. § There were a number of people who had wanted the collection. The question was: had anyone wanted it badly enough to kill Fleming? And if so, how had he done it? Here is a mystery, told against the fascinating background of old guns and gun-collecting, which is rapid-fire without being hysterical, exciting without losing its contact with reason, and which introduces a personable and intelligent new private detective. It is a story that will keep your nerves on a hair trigger even if you don't know the difference between a cased pair of Paterson .34's and a Texas .40 with a ramming-lever."... one free source:
http://www.gutenberg.org/ebooks/17866
The Rome Express by Arthur Griffiths. Synopsis: "A mysterious murder on a flying express train, a wily Italian, a charming woman caught in the meshes of circumstantial evidence, a chivalrous Englishman, and a police force with a keen nose for the wrong clue, are the ingredients from which Major Griffiths has concocted a clever, up-to-date detective story.". One free source:
http://www.gutenberg.org/ebooks/11451
The Thirty-Nine Steps by John Buchan. Synopsis: "When Richard Hannay is warned of an assassination plot that has potential to take Britain into a war, and then discovers in his flat the murdered body of the American that warned him, he becomes a prime suspect. He flees to the moors of Scotland and a spirited chase begins as he is pursued by the police and the German spies involved with stealing British plans. Buchan's tale unfolds into one of the seminal and most influential 'chase' books, mimicked by many, yet unrivaled in the tension and mystery created by his writing.". One free source:
http://www.gutenberg.org/ebooks/558