"Passchendaele - The Anatomy of a Tragedy" by Harper Collins is a good overall read, and at 250 odd pages not too encompassing.
"Passchendaele - The Untold Story, 2nd Edition (Yale UP)" is an excellent assessment of the third battle of Ypres.
"Magnificent But Not War - The Second Battle of Ypres 1915" by Pen & Sword is very accessible.
"Massacre of the Innocents - The Crofton Diaries, Ypres 1914-1915" is a diary of Sir Morgan Crofton, who took part in the First Battle of 1914. Interesting perspective.