About 4,100 pages of the war poet's personal archive have been digitised for the Cambridge University Library project, which aims to highlight the horrors of the First World War.
They include draft copies of his Soldier's Declaration as well as poetry, prose and sketches.
Sassoon served on the Western Front but was sent to Craiglockhart War Hospital in Edinburgh in 1917 for speaking out against the war. He was declared unfit for service due to apparent shell-shock and was sent to the hospital to convalesce, famously meeting fellow poet Wilfred Owen there.
He later returned to the frontline.
The poems include previously unpublished material along with early drafts of some of Sassoon's best-known works including an early version of The Dug-Out with an additional, excised verse.
The notebooks were small enough to be carried in the pocket of his army tunic, and he used them to draft poetry, make pencil or ink sketches, list members of his battalion and their fates.
Cambridge University librarian Anne Jarvis said: "The Sassoon archive is a collection of towering importance, not just to historians, but to anyone seeking to understand the horror, bravery and futility of the First World War as experienced by those on the frontlines and in the trenches."
lSee Monday's Herald for a 28-page First World War supplement.