The mission as a frontier institution in the Spanish american colonies, by Herbert E. Bolton.

http://www.archive.org/details/missionasfrontie00boltrich

Frontiers of the Christian world mission since 1938; essays in honor of Kenneth Scott Latourette. Edited by Wilbur C. Harr.

http://www.archive.org/details/frontiersofthech013243mbp

The Spanish pioneers and the California missions.

http://www.archive.org/details/spanishpioneersa009682mbp