Seven slim volumes that were the journals / workbooks of an artificer who specialized in cannonry.
The journals are not written as a cohesive narrative but rather consist of working notes, diagrams, and the records of various experiments and attempts at creation during the course of Kendrick's career. As a result it is impossible for anyone that is not a fellow Artificer to make much sense of them.
However, Kendrick was very skilled at his craft, and when read by a likeminded individual, several interesting paths of research and creation are outlined.
Most notably, Kendrick was very interested in constructing cannons out of alloys of mithril and adamantium, and spent a large part of his life perfecting the process of metallurgy and manufacture of such cannons. From his notes, it is clear that these devices offered significant advantages over normal iron or brass cannons, or even over cannons composed entirely of mithril (pure adamantium cannons are taken to be impossible as there is no known method of drilling out the barrel). They were also ridiculously expensive. Kendrick died penniless and without fully achieving his goals due to the extreme cost his methods incurred.
The other major area of research Kendrick engaged in was tempering his creations in an alchemical concoction that included the blood of various supernatural creatures. He records several successful experiments where the cannons themselves inherited supernatural properties from the creatures from which the blood was acquired.
While these books are not detailed instructions or blueprints, but rather workbooks, and hence have some omissions and take certain parts of the process for granted, it should be possible for other artificers to recreate Kendrick's accomplishments, with some trial and error.