There were historically two times that the Brits toppled pro democratic govts in Iran and reinstalled monarchist rule

Most people know about 1953: after the Iranian Prime Minister Mossadegh nationalized Iran's oil industry ( which until then was operated as a concession by British Petroleum, and powered the British navy), a joint CIA/ MI5 operation known as TP AJAX was put into effect, reinstalling the last Shah of Iran (the same one that would be later permanently toppled in the 1979 Islamic Revolution) Currently the most popular book on that is "All the Shah's Men" by Stephen Kinzer. http://www.amazon.com/All-Shahs-Men-American-Middle/product-reviews/0470580410

However I also recommend Prince Farmanfarmaian's book "Blood and Oil"

What most people don't know is about the 1906 Constitutional Revolution in Iran, which established the first democracy in the Mideast, ended the 2500 history of absolute monarchy in Iran by adopting a system of Constitutional Monarchy where the King reigned but an elected Parliament ("Majlis") ruled as in the UK ( actually they copied the Belgian constitution). However this was short lived, since the British and Russian imperial forces backed another shah and put him back in power, and ended up shelling Iran's Parliament building. http://www.iranreview.org/content/Documents/Contemporary_History_The_Bombardemant_of_Majlis_June_23rd_1908.htm

Probably the best source on that is Janet Afary's book " The Iranian Constitutional Revolution"

A certain American named Morgan Shuster who had been hired by the democratic govt to manage Iran's finances (who Britain and Russia hated) wrote a famous memoir about the times entitled "The Strangling of Persia"

All of these books are available on Amazon, buy used I suggest.