2. General: "The Use of Underlayment with Asphalt Shingles" explains why you use it. Available at the Canadian Asphalt Shingle Manufacturers' Association website.
3. Building Codes: "R905.1.1 Underlayment" from the 2015 International Residential Code. Available at the UpCodes website. For underlayment sections in the 2015 International Building Code, you have to look under the subsection for each specific roof covering, beginning with "Section 1507 Requirements for Roof Coverings".
4. Technical: "Grace Ice & Water Shield HT" is a product sheet worth taking a look at for information on this very popular underlayment. Includes product description, guidelines for use, and installation procedure. Available at GCP Applied Technologies.