I suppose it depends on how good you know Delphi and whether you are aiming for a distinction or not?
IMHO, if you already got the knowledge of every thing that is covered in the study material you can probably write the exam and pass.
The question is how would you know if you got all the knowledge, if you don't know what is in the book.
Personally I find most of the 1st year study material "entry level" cause I got 8 years experience already, but every know and again there is actually something new that I learn.
There is no need to buy the book new, I bought mine second hand for R200. If you learn 5 new things in the book, that is not covered by the assignments it's probably worth it.
PS. not everything is covered in the assignments, so if you do the assignments on your own research it doesn't mean you've covered all the work.