In 1769, he was promoted to captain and published a small book about training and drilling the militia. This book was published in 1775 as "An Easy Plan for a Militia" and was used as the training manual for the Continental Army until it was replaced by Baron von Steuben's new regulations in 1779.
This letter was written to General George Washington, apparently accompanying Pickering's book.
From Timothy Pickering
Sir, Convinced of the utility, the necessity, at all times, of a well disciplined militia, to every free state; when the united wisdom of the continent, referring to the contest with the parent kingdom, called on every colony to prepare for the most unhappy events; and the more immediate recommendations of our provincial congress demanded a diligent application to the military art; deeming the plans of discipline then extant, inadequate to the instruction of men unused to this kind of study, & destitute of living instructors: I gladly embraced the opportunity which then presented, of applying to the service of my country the little knowledge & experience an office in the militia had led me to acquire, by writing the following plain rudiments of the military art.
They were designed, as their title imports, merely for the militia; & chiefly written before the predicted unhappy events had called my fellow citizens to arms. This call, & the various avocations & interruptions consequent thereon, greatly retarded the completion of the work; & perhaps have rendered it less useful than it might otherwise have been: Some parts of it, & those perhaps the most essential, I imagine may well prove advantageous in an army hastily assembled, & frequently called from the exercise of arms to the other equally necessary but more laborious occupations of war.
This army being, to the joy of every American, committed to your excellency’s care & direction, both duty & inclination lead me to present you the ensuing plan of discipline for a militia, & to Submit to your decision the expediency of recommending or permitting its use among the officers & soldiers under your command.
I am, your Excellencys most obedient & humble Servant
Tm. Pickering jr