Yes, the Lectitio Divinatus. It was written by the Primarch of the Word Bearers, Lorgar Aurelian, before he fell to chaos. The Emperor then forbid it but it was already out there and people began to worship the Emperor as a god.
That is also one of the reasons that it is not very widely known that Lorgar is a daemon prince of chaos undivided, as his book is the basis of the Imperial Faith.
This faith unified the Imperial people against the myriad threats and horrors of the galaxy in their common belief that "faith in the Emperor protects" until the cult of the Lectitio Divinitatus became the official state religion of the Imperium in the 32nd Millennium in the form of the Temple of the Saviour Emperor.
From the Fandom article on the Lectitio Divinatus
There are also other books, that can be used:
Book of Saint Lucius
Few orators in the history of the Imperium have equalled the fiery rhetoric of Saint Lucius of Agatha. The sermons of the famed Confessor have been published and republished countless times on thousands of the worlds across the galaxy. The righteousness of the revered saint's words echo through the long centuries and continue to inspire the faithful to greater acts of devotion.
From the Rogue Trader rulebook Faith and Coin, page 97
Also known as the "Book of Hours", the Thorian Missal is the illuminated book containing prayers, liturgies, sermons and a calendar of Imperial feast days. This collection of writings is attributed by some to Sebastian Thor, an ancient holy man who came into his ascendency during a time of great conflict in ages past.
Above mentioned rulebook, page 101