You can call the number for the California Franchise Tax Board (800) 852-5711 to verify.  Do not use the number on the letter, you want to verify its authenticity before sending it in.  However, it seems as if you were randomly chosen to have your information verified before they process your return.  States do this from time to time to help deter identify theft. 

I also got the letter and was concerned about it's legitimacy.  If you have an online account with California's Franchise Tax Board, you can login and see your letter along with the document cover sheet and confirm that the request is legit.