MARK TWAIN'S new book is to be all about his experience in California and Nevada and coming to this coast by the Overland stages. It will be a volume of 600 pages octavo. He has started in on it, but at last accounts was two days' journey the other side of Salt Lake with it; so we need not get excited here about the "chief amang us" for some time to come. His book will be out in April next.