This is one of the best of Dick Francis's mystery novels; if you've never read him, this would be an excellent first pick. (It's one of the few Francis books apparently not now in print, but should be easy to find at a second-hand shop or your local library.)
Jockey Derek Franklin is nursing a badly broken ankle from a racing fall. Even more bad news follows, as he gets news that his older brother Greville has been critically injured in a construction accident. Greville's passing means that Derek must decide to do with Greville's gemstone business, and also the racehorses Greville owned. That puts him in over his head, but things get much worse: Derek is accosted (numerous times), and Greville's workplace and home are burgled. And a bunch of uncut diamonds have gone missing.
Derek needs to unravel Greville's business and personal affairs to discover who's behind all the evildoing. Watching him do so is a delight.