This site offers access to over 150,000 publications written by USGS scientists.
The American Geological Institute maintains a database of geologic field trip guidebooks. Some are available online, but many are not. If the guidebook is available online, you'll see a "Get Full Text" link. You can narrow your search on online results by clicking "online" on the right side of the search results. Some of those links will take you to a site that requires you to pay for the guidebooks. You can request these through InterLibrary Loan, though you will need to get your requests in quickly and they may not arrive in time. Make sure you only request a chapter from the guidebook, not the whole book (a chapter will be sent electronically as a pdf, the whole book will be sent via the mail and will not arrive in time)