Only, alone are often used interchangeably (though alone is not found in the attributive position), but seldom without a slight change in meaning or emphasis.
Only is especially appropriate when restriction to what is specified or asserted is implied and the term is equivalent to sole or solely; thus, “I want only this book” implies a wish for one and no more; “of all the family only John and Helen came,” that is, the specified persons and no more.
Alone may be chosen when the idea of the elimination of all other possibilities is expressed and the term is the equivalent of exclusive or, more often, exclusively; thus, “I want this book alone ” implies a wish for a particular one and no other; “of all the family John alone came,” that is, John and none of the others.