The Digital Public Library of America has made on open access a collection of paper s relating to the impeachment of Donald Trump. They also include materials relating to alleged foreign interference in the 2016 presidential elections.
Background information on the nature of the impeachment process in the USA and how it has been used in the past. An excellent respected source are the EveryCRS reports which contain Congressional Research reports prepared to inform members of the Congress. All can be downloaded in full text.