The American Presidency is a good example of where the Very Short Introduction series by Oxford Press can be helpful. At just over 150 pages of real content, this book can be read in a long sitting and give some real background to the subject area.
When the Very Short Introduction series gets it right, the books are usually overviews with a couple of main points. When they get it wrong, the books usually focus more on the research around the subject and forget to actually introduce the subject itself. The American Presidency is one of the former.
The main focus of the book is how the office of the President relates to the rest of the US government, which requires a short introduction to all three branches. A balanced government, with no branch dominating, was an innovation when the Constitution was written. The term President is based on the idea of presiding over something. Governor would have been a more accurate idea of the type of office envisioned, but that office had a negative connotation because of the appointed Governors prior to the Revolution.
The office of the President executes the functions of the federal government. As the country has grown in both size and complexity, the size of the government has grown and the complexity of managing a workforce of about 3 million people. (The size of federal employment has varied, but it currently approximately the same as 1967 in real numbers. Although the federal government now uses significantly more contractors, which are not included in the employment figures.)