I never knew my grandfather but he served in the first world war and the book is called The Daily Portion Testament. I'm guessing it was issued to troops to give them comfort and to help preserve faith.
This is inside the front cover:
Lord Roberts's Message to the Troops
25th Aug 1914The message inside the front cover is not overly legible which is not surprising, as if it was Lord Roberts that wrote the message by hand, he was 81 at the time. He died later that year, in November, aged 82.
I ask you to put your trust in God. He will watch over you and strengthen you.
You will find in this little Book guidance when you are in health, comfort when you are in sickness, and strength when you are in adversity.
I wonder if the same time, thought and effort goes into the spiritual welfare of today's troops.