The revised version (edition number 2) of the Jepson Manual: Higher Plants of California is now in print. It is a comprehensive flora of the nearly eight thousand taxa of native and naturalized California plants. It includes significant revisions to plant families, genera and species. It is available from the UC press for $100 at http://www.ucpress.edu/book.php?isbn=9780520253124. It is 1600 pages and 7.4 pounds so though a great resource you may not want to carry it very far in the field.
It is also available in an interactive digital version at Jepson eFlora. The on-line version will have regular updates.