This'll Slay You (1958)

Alan Payne
Ace Double D-289

Fairly by the numbers pulp of two low-rent gamblers, fatty and skinny, looking to retrieve an envelope containing valuable information about the ponies.  When an obnoxious vet student and little brother of the busty Gloria is murdered in Florida, the two gamblers head to Indiana in search of answers.  There they discover a scientist has found a way to predict how well horses will do in a race based on their blood count--valuable information to have in advance of a big stakes run coming up in Louisville. 

Lots of shooting, drinking, fleeing, fighting, etc., all peppered with wisecracks between the gambling tandem as they get in and out of scrapes.  In a comic centerpiece, the gamblers hypnotize a stupid small town sheriff and give him "negative hallucinations" (i.e. the gamblers become invisible to him, thus allowing them to escape).

Alan Payne was an early pseudonym for John Jakes, who in the 1970s became a best-selling author by writing sprawling works of historical fiction, most famously The Kent Family Chronicles (The Bastard '74, The Rebels '75, and many more) and the North and South trilogy.  These later books, as any used book store employee will tell you, sold enough copies to damn the mightiest rivers of the world several times over.