As relative pronouns, these words are often used interchangeably. Careful writers and speakers, however, use that to introduce restrictive (defining) clauses and which to introduce nonrestrictive (nondefining) clauses. So used, which should always be preceded by a comma (or a pause); that, never. In "This is the book that my son gave me for Christmas," that my son gave me for Christmas is a restrictive clause that (not which) defines the book. In "This book, which is by Richard Lederer, was given to me by my son," which is by Richard Lederer is nonrestrictive. It gives additional information about the book without defining it as a particular book.