A Farewell to Arms by Ernest Hemingway (1929). Hemingway drove ambulances on the Isonzo Front in 1918 and drew directly on that experience for this novel. Lieutenant Henry's retreat from Caporetto — the Battle of Kobarid — is one of the most vivid pieces of writing about the chaos of military collapse. Reading it before visiting makes the landscape and the Kobarid Museum resonate very differently.
The White War: Life and Death on the Italian Front 1915–1919 by Mark Thompson (2008). A detailed and unsparing history of the Italian campaign on the Soča/Isonzo, covering the eleven Battles of the Isonzo in sequence. Thompson's research in Italian and Austro-Hungarian sources makes this the most thorough account in English of what happened in this valley.
Alamut by Vladimir Bartol (1938). Not directly about the Soča Valley, but this Slovenian novel — a fictional account of the Assassins of medieval Persia — is one of the most significant works of Slovenian literature and worth reading to understand something of the Slovenian intellectual tradition.