I had a few hours to kill today so I skimmed through the 693 pages of The Reagan Diaries.
This is a good book if you want to know more about Ronald Reagan, the man, the husband, and the father. However it doesn't provide you great insight into Reagan, the President.
The reader will discover that Reagan obviously adored his wife and had a genuine good heart for friends and people he met that had needs.
You'll learn a bit about his White House schedule, who he met and maybe some brief capsules of conversations in those meetings.
My sense is that Reagan's diary entries were not unlike how he governed--not involved with a lot of detail.