Just finished reading "Duty" by former Secretary of Defense Robert Gates. He is the only Secretary who served two different political parties; George W. Bush and Barack Obama.
He's a self-declared moderate Republican, but he goes into detail about the Iraq and Afghan wars, and how he comes to respect both Presidents despite their very stark differences/approaches to war. I don't get too much of a right-leaning vibe at all from the book. He trashes Congress early and often, and talks about the bureaucratic barriers and bullshit he had to put up with at the Pentagon to get anything done to aid the troops and war vets.
It's a thick read but if the Middle East wars/DoD interests you, it's really fascinating. He goes into a lot of behind the scenes detail about WH staff members for both 43 and 44, members of the Military, etc. He most definitely burnt some bridges if he hadn't already before retiring, lol.
Interesting note, he mentions in the book (wrote in 2014) that he butted heads the very most with Joe Biden and they have very stark differences in how to go about foreign policy. He said he has basically disagreed with every foreign policy stance Biden has had up to the writing of the book. He got along just fine with 44, but Biden was often in Obama's ear singing a different tune, before, during and after meetings with his Defense staff.