Impeachment Grounds: A Collection of Selected Materials [Updated October 29, 1998] [open pdf - 142KB]
From the Summary: "This is a sampling of the material available on the question of what constitutes impeachable conduct for purposes of Article II, section 4 of the United States Constitution. It is arranged in chronological segments. The first covers those materials available on impeachment at the time the Constitution was drafted, [...] The second consists of the debates at the Constitutional Convention and the state ratifying conventions. The third includes explanations from those who participated in the Convention and their contemporaries. [...] The fourth is a summary of major American impeachment cases under the Constitution. [...] The fifth is made up of material prepared at the time of the Douglas and Nixon impeachment inquiries including then-Congressman Gerald Ford's remarks. The collection ends with a very limited assembly of scholarly views from treatises and law reviews."
CRS Report for Congress, 98-882 A
Congressional Research Service: https://crsreports.congress.gov/