The book argues that peace and stability in the state depends on how a state is governed. The author argues the state should be governed by sharia law and ‘good/just governance’ (in modern parlance). If the state governance and offices are filled with incompetent people, if they wield too much powers and grab many posts, there will be injustice and chaos and the ruling elite will fall. In fact, he complains a lot about the corruption and many incompetent people in Saljuk empire's government.
The essence of his book is that justice and laws should prevail and no one life, property should be violated. The Sultan’s job is to protect its subjects against such violations. In my reading, it puts too much emphasis upon the ‘personal qualities’ of the prince rather than the superiority of laws (sharia law) or institutions, which is somewhat understandable.
There are many points in Siyasatname which are similar to Machiavelli’s Prince (e.g. how should prince behave, what principles he should follow, etc). However, unlike the Prince, it is does not give ‘strategic advices’ as to how to hold on power, etc. Having said that, the book did not sound a moralistic book to me at all. It describes many stories where the king (sultan) kills his opponents (political, religious, etc) in mass, which should be an exemplary behavior in similar situations.
Religion (Sunni Islam) is the essence of the state and the state should not allow any (unconventional) major religious sects to spread such ideas as this would end in state’s collapse. He did not advice giving women, religious minorities (Christian, Jews, Shia, etc) a major role in government, if there are competent Muslims to fill up such jobs.