According to the Gregg Reference Manual: "To form the possessive of a singular noun that ends in an s sound, be guided by the way you pronounce the word. If a new syllable is formed in the pronunciation of the possessive, add an apostrophe plus s."
Ex: your boss's approval (you pronounce it as bosses); therefore, add the apostrophe.
Also according to the Gregg Reference Manual: "However, if the addition of the extra syllable would make a word ending in s hard to pronounce, use the apostrophe only."
Ex: Los Angeles' freeways (try saying Los Angeleses). Difficult to do; therefore, just use the apostrophe.
Best grammar books are The Gregg Reference Manual and the Simon & Schuster Handbook for Writers.
I have no problem with boss' approval. At least this is a rule where one can understand why people get it wrong. There are grammar errors that bother me more; I cannot understand why people misuse apostrophes.