Edwin Bryant's What I Saw in California is an essential part of American history, of particular interest to California and the westward expansion. Bryant's connection to the infamous Donner Party adds more appeal to his book, which was often used as a trail guide by those traveling west in the late 19th century.
Perhaps surprisingly, considering the initial trail-guide usage of this book, there are first edition copies available for collectors to purchase for only a few thousand dollars. These are usually in what would be considered less than optimal condition for other books, but considering how old the book is and its use as a field guide in many cases, any first edition copy should be valued.
Some of the other copies of What I Saw in California that are considered collectible are later editions, which were published repeatedly throughout the 1850's. These are valued between $200 and $500, depending on publish date and condition. In the early 1900's, nice hardcover editions intended for collecting began to be printed. These are also collected by Bryant admirers, though they rarely sell for more than $200.
The fifth edition of this book was the first to include a map, so copies of the book that still include this folding map are considered quite valuable, with a 2015 auction for one ending at $1,140. In some cases, What I Saw in California was printed bound with other travel- and exploration-related books in the years immediately following its initial publishing. Depending on what those books are and when they were printed, these can substantially add to the value of the book.