Double Star by Robert Heinlein is $2.99 at Amazon.
The story revolves around Lawrence Smith—also known as “Lorenzo the Great”—a down-and-out actor wasting the remainder of his life in bars.
When he encounters a space-pilot who offers him a drink, before he knows what is going on, he is on Mars involved in a deep conspiracy with global consequences. He is given a mission where failure would not only mean his own death, it would almost certainly mean an all-out planetary war.