The Gallagher Law Library guide on Finding Washington State Laws includes information about sources for historical laws.
The Washington State Legislature's Glossary of Legislative Terms.
Other libraries have guides on Washington legislative history:
Legislative History - Washington State Archives
Legislative History Guide - Public Law Library of King County
Washington Legislative History and Legislative Intent - Washington State Law Library
Need to research legislative history for a state other than Washington? See State Legislative History Research Guides Inventory - Indiana University Jerome Hall Law Library for links to comparable guides prepared by other law librarians.