I think you are reaching one of the limitations of the English language here. Machiaveli's Prince and John Knox's Monstrous Regiment of Women are both "political" books, but in a very different sense. The former is an exercise in trying to understand the nature of power and society in specific circumstances (in particular, the Prince is a study of autocratic power by a committed republican). The latter is just a polemical weapon, designed to advance some political goal. When people complain about politics in literature, it's usually because they don't like reading the second sort of book. That sort of books are seldom good, whatever their genre may be.
(I'm intentionally using Renaissance examples here, to avoid any unproductive discussions on more modern books.)