The House of Commons Parliamentary Papers (HCPP), also known as the Sessional Papers or Blue Books (after their bindings) are the collected papers of each session of the House of Commons of the United Kingdom. The consist of bills, documents relating to the work of the House of Commons and "Command Papers": papers and reports presented to the House by the government.
Every session of Parliament has a set of volumes published containing the papers of that session. Sessions generally cover a single year, but there can be more than one session a year, and a single session can span more than one year. There are now thousands of volumes in the HCPP.
The last volume in each series contains index of the papers in that series. Command papers are generally not numbered in the main text, but are assigned numbers by this index.