How do I access files in a Shared Library?

stephenking's answer is more complete but for simple cases the following will do:

writeFile file: 'ps1FileInMySharedLibVarsFolder.ps1', text: libraryResource('ps1FileInMySharedLibVarsFolder.ps1')
powershell ".\\ps1FileInMySharedLibVarsFolder.ps1"

You can only get the contents using the built-in step libraryResource. That's why have the following functions in my shared library to copy it to a temporary directory and return the path to the file:

/**
  * Generates a path to a temporary file location, ending with {@code path} parameter.
  * 
  * @param path path suffix
  * @return path to file inside a temp directory
  */
@NonCPS
String createTempLocation(String path) {
  String tmpDir = pwd tmp: true
  return tmpDir + File.separator + new File(path).getName()
}

/**
  * Returns the path to a temp location of a script from the global library (resources/ subdirectory)
  *
  * @param srcPath path within the resources/ subdirectory of this repo
  * @param destPath destination path (optional)
  * @return path to local file
  */
String copyGlobalLibraryScript(String srcPath, String destPath = null) {
  destPath = destPath ?: createTempLocation(srcPath)
  writeFile file: destPath, text: libraryResource(srcPath)
  echo "copyGlobalLibraryScript: copied ${srcPath} to ${destPath}"
  return destPath
}

As it returns the path to the temp file, you can pass this to any step expecting a file name:

sh(copyGlobalLibraryScript('test.sh'))

for a file residing in resources/test.sh within your shared library.