The Internet Archive - Texts
Ignore this resource at your peril; it is one of the richest sources of free WW1 contemporary material on the internet.
The bottom of the ‘Advanced Search’ window contains guidance on forming your queries. Although I use the advanced search when interrogating specific fields I tend to stick to the simple text search.
My search strategy is fairly simple:
Tanks, 1914-1918; the log-book of a pioneer (1919)
Life in a tank (1918)
A company of tanks (1920)
The tank in action (1920)
The tale of a tank, and other yarns ([1917?])
This time you are on your own............ try and find ‘Sir Douglas Haig's despatches’, and ‘William Robertson’s autobiography’.
Haig "1914-1918" would have given you:
Sir Douglas Haig's despatches (December 1915-April 1919) (1919)
Robertson “1914-1918” would have given you:
From private to field-marshal (1921): Robertson, William Robert
How about Field Service Regulations Part One?