The official reason for the book's shortlisting - given the three-year period between publishing and prize - is currently unclear.
But it is understood that new RIBA library chief Irena Murray campaigned for the book to win because 'it was too good to go unrecognised'.
As a result, the book picked up the award - which was instituted in 1954 by Banister Fletcher - at an event at London's Authors' Club.
Balmond's book comprises notes, sketches, diary extracts, photographs and CAD drawings, documenting his work with Rem Koolhaas, Daniel Libeskind and Alviro Siza.
Speaking of the award, Koolhaas said that Balmond was a pivotal figure in the development of engineering.
'Balmond has, almost single handedly, shifted the ground in engineering - a domain where the earth moves very rarely - and therefore enabled architecture to be imagined differently,' he said.