The return value is printed readily, that is, using a view that can be parsed using READ into the CL object. If you use a function like PRINC that prints the string as is, you will see that the quote did what you wanted (the external quotation marks are not part of the string):
CL-USER> (princ (concatenate 'string "Mama said: " "\"Son, your life is an open book...\"")) Mama said: "Son, your life is an open book..." "Mama said: \"Son, your life is an open book...\""
The first line is the result of PRINC, the second part of the PRINT part of the READ-EVAL-PRINT-LOOP.