Manuals and Guides:
Library of Congress. Map Cataloging Manual, 1991 edition. This manual is currently being rewritten. Available in Cataloger's Desktop.
AACCCM. Cartographic Materials: A Manual of Interpretation for AACR2, Second Edition, 2005 update. This manual is out of print, and the AACCCM has been disbanded. There are no plans to rewrite or update it. Available in Cataloger's Desktop.
Andrew, Paige. Cataloging Sheet Maps: the Basics, 2003. A very readable guide to the intricacies of map cataloging, but only covers print sheet maps, not atlases, globes, or maps in electronic or other formats.
Andrew, Paige G., Susan M. Moore, and Mary Lynette Larsgaard. RDA and Cartographic Resources, 2015. Primarily a guide to the differences RDA makes in map cataloging, not a complete map cataloging guide.
Tools:
Klokan's Bounding Box tool, http://boundingbox.klokantech.com/. Search for a location; it will show the bounding box and return the 255 |c and 034 |d, |e, |f, |g in a variety of formats. Select "MARC OCLC" in the dropdown at the bottom, and copy and paste the result into an OCLC catalog record.
Business Map Centre's Scale Buddy: http://www.businessmapcentre.com/scale-buddy.php. Select the "Calculate Scale" option. Enter a measurement from the map ("Map Distance") for a feature of known real-world size ("Ground Distance") and click "Calculate" to get the representative fraction. Bar scales are an obvious object to measure, but township, range, and section lines work very well for this, since a section is (usually) 1 mile square.
The Newberry Library's Atlas of Historical County Boundaries: http://publications.newberry.org/ahcbp/. Contains lists of changes to county boundaries for every state. Good information for dating undated historical maps.
Geographic Names Information System: http://geonames.usgs.gov/. Authoritative source for place names.
Road Map Collectors Association: http://roadmaps.org/. This is primarily a members-only site, but it has some very good publicly accessible information as well. The page on dating Rand McNally and Gousha maps is especially useful.
AA Roads' Wyoming Highways site: http://www.aaroads.com/west/wyoming.html This site gives information on the construction history and routes of interstate, U.S., and state highways in Wyoming. It can be very useful for approximating the dates of undated maps that show these highways. Sites exist for other states as well, and for highways that run through more than one state.