Cassell's recommended usage is British. It is not accepted in American publishing.
"The American Heritage Book of English Usage" is a current book of accepted practice.
English Usage

Recommended-
"The Chicago Manual of Style," 2003 ed., compiled by the staff of the University of Chicago. The American Library Association, in a review, says "Chicago" offers "sensible, clearly articulated and defensible advice to authors and editors who want to do their best to present an author's text to readers."
I would add that the book will help anyone who does a lot of writing.
Available from any book dealer, and at $36 from Amazon; the 14th ed., available for very little, 1993 date, would probably be sufficient for most purposes.

Note- UK writers have their choice of Fowler's or Cassell's.