The Gazetteer of the Persian Gulf, Oman and Central Arabia (nicknamed Lorimer)  is a two-volume encyclopedia compiled by John Gordon Lorimer. Published in secrecy by the British Raj government in 1908 and 1915, it served as a handbook for British diplomats in the Arabian Peninsula and Persia.
Declassified in 1955 under the fifty-year rule, it was widely praised for its extensive details of the region's history and geography. It is considered to be "the most important single source of historical material on the Gulf States and Saudi Arabia" from the 17th to early 20th century.
At the turn of the 20th century, the British empire sought to solidify its links to British-controlled India which in turn resulted in a greater interest in the Persian Gulf region, culminating in the visit of the Viceroy of India Lord Curzon to the Gulf in 1903.  To ensure that British agents in the region were adequately informed and prepared to strengthen their influence in the region, a convenient and portable handbook was needed. 
The British Raj commissioned John Gordon Lorimer, a member of the Indian Civil Service serving in the North-West Frontier Province, to compile such a document in November 1903. Initially given six months to complete the task, Lorimer repeatedly insisted on being granted more time to ensure the work was completed thoroughly. Placed under special duty and with a team of researchers, over the next 10 years, data was collated from government archives in Calcutta and Bombay, and from multiple field expeditions to the Gulf.
The Gazatteer itself:
The gazetteer is a 5000 page document divided into two volumes; the first details the region's history and the second details its geography. The geography portion of the gazetteer was completed first and was published in 1908. The history portion of the gazetteer was only completed and published in 1915, a year after Lorimer himself died in a shooting accident.
Entitled 'History, geography and geneaology', the volume was split off into three sections which were divided by ethnicity. Section 1 (the Arab section) was dedicated to the history of the Persian Gulf, central Arabia and Ottoman Iraq. Section 2 (the Persian section) regarded the history of Persia with particular interest to the predominately-Arab populated region of Arabistan. Section 3 consisted of 19 genealogical trees of the ruling families in the region. 
Its research was compiled from Lorimer's own notes and colleagues such as J.A. Saldanha and C.H. Gabriel and covered various periods ranging the 17th to 20th centuries. 
Entitled the 'Geographical and Statistical' section, it is a 2000 page document and lists an extensive alphabetical arrangement of tribes, towns and villages across the region, divided into different countries.
This data was obtained through field trips and surveys conducted by Lorimer and his team. Also in the volume are 56 reproduced images of the region taken from colonial records and two maps showing the distribution of pearling sites and the overall political geography.
|Map of the Persian Gulf, Oman and Central Arabia, compiled by Hunter (IOR/L/PS/20/C91/6, f. 1r)|
In 1971, The Times Literary Supplement praised the document, regarding its historical coverage as "stupendous" and its geographical section as "without modern substitute". The gazetteer, relying on British sources and written from a British viewpoint, is regarded as a valuable resource in researching the history of the Gulf. 
The gazetteer in its entirety was digitised and uploaded online by the Qatar Digital Library in January 2015.
Further reading and references:
"Gazetteer of the Persian Gulf, Oman and Central Arabia". Cambridge Archive Editions
Lowe, Daniel A. "‘Persian Gulf Tragedy’: the Death and Legacy of John Gordon Lorimer". Qatar Digital Library. Retrieved 10 August 2015.
Leech, Nick (7 January 2012). "A reference book for every historian to rely on". The National.
Lowe, Daniel. "Colonial Knowledge: Lorimer’s Gazetteer of the Persian Gulf, Oman and Central Arabia". Qatar Digital Library. Retrieved 10 August 2015.
Lowe, Daniel (6 December 2014). "The diplomat’s portable handbook (wheelbarrow required)". BBC News. Retrieved 10 August 2015.
Author's note: the above was previously posted onto Wikipedia first.