Steve Vogel's book "The Pentagon: A History" provides a fascinating insight into how military projects are funded and the changes which occur in the development of a project. Also the way the building was designed and constructed is of interest. Some of the history of the construction of the building was repeated in its refurbishment after the terrorist attack on 11 September 2001.
Some of the history I found of particular interest was that the pentagonal design of the building derived from the shape of the block of land it was originally planned to be built on. After the location was changed the design was retained, even though the original rationale for the shape was no longer there. The building was designed and constructed as five units. The plans for the units where partly reused on subsequent units.
The building was originally planned as a temporary headquarters during World war 2. To justify such a large and clearly non-temporary building, it was strengthened to be able to be used as the government archive after the war. The result was that the building was much stronger than needed for offices. This, and reinforcement installed shortly before 11 September, resulted in less damage and fewer casualties than would otherwise be the case. It is surprising that conspiracy theorists made nothing of the fact that the aircraft on September 11 struck the centre of the only wing of the building which had been strengthened against attack, that the work had only just been completed and the attack occurred before all the staff had moved back in.
One piece of trivia which Steve Vogel might like to put in a later edition is that Washington is not the only capital to have a pentagon shaped national military headquarters. The Defence Headquarters at Russell Offices, in Canberra, is made up of two five sided buildings (with central courtyards like the Pentagon). These are appropriately located just behind the Australian-American Memorial.