1. This book is the most unique of all.
  2. This book is the unique of all.
  3. This book is the/a unique.
  4. This book is unique.
    Please check above and comment.

HI Fathima

As for my knowledge is concerned, The No.4 is correct because the word Unique is already different from all. when we say She is unique-it makes sense of she is different from all, So no need to mention all and the most after or before it.

Abbas Khan

Can I say :
“It is a unique book”
“He is a unique personality.”

I hope you can help me.