To your list of Lassen material I would add:

Ernest P. Peninou's "Leland Stanford's Great Vina Ranch 1881-1919: a research paper: the history of Senator Leland Stanford's Vina Winery and the world's largest winery formerly the site of Peter Lassen's Bosquejo and Henry Gerke's Ranch" (San Francisco: Yolo Hills Viticultural Society, 1991). It deals with Lassen from the perspective of the ranch before getting on to what he is really interested in.

And then there is T. Vogel-Jorgensen's "Peter Lassen af California" (New York: Arno Press, 1979). It is in Danish and essentially worthless to us Danish-challenged researchers. I had the good luck of discovering a Danish-schooled exchange student from South Africa living with my sister at just the time I was wrestling with the question of whether Lassen knew Sutter in Missouri. I learned from him that the book is actually in a rather archaic Danish, but that it did state that Lassen had met Sutter in Missouri. Which is well and good, but I hardly imagine Vogel-Jorgensen knew any more about it than I do.

Wendell.