A Change of Gravity - George V. Higgins

For court clerk Ambrose Merrion, life was about people taking care of one another. For Danny Hilliard, politics was a matter of acquiring the power to make sure that society did just that. With Merrion shrewdly managing Hilliard's campaigns as a rising politician, the two friends made an excellent team.
But trouble starts brewing when Merrion unexpectedly inherits ill-gotten gains from a corrupt predecessor, enabling him to indulge with Danny in the finer luxuries. To Merrion's dismay, Hilliard begins to show a streak of mischief that quickly flares into a scandal. And a federal prosecutor on a mission believes he has found an the perfect way to put Hilliard in jail—forcing Merrion to incriminate him. For the rules have changed while the friends weren't looking, and what was once just common wickedness has become a felony.