If you’d like to find out more about 58th Bde, RFA, I would recommend the following:
Details of the brigade’s service overseas may be found in their War Diaries, available from The National Archives under references WO95/4298, WO95/1800(1-5) and WO95/1801(1-4).
Letters written by Robert Bragg during his service in A/58 are available from the Royal Institute’s website, under the heading “The Bragg Family in WW1: Selected Correspondence”
“Broken by Messines” by Dr Mark Wardlaw, tells through letters from A/58 soldier, Peter Hutchinson Wardlaw, the love affair between him and his future wife and how injuries he sustained in action blighted their futures.
“Medal Without Bar” by Richard Blaker, is a semi-autobiographical account of Blaker’s experiences as a subaltern in D/58 from July 1916 until July 1917.
Chris Baker’s website, “The Long Long Trail” is a very valuable resource giving details about all of the units in the British Army in the Great War.