When do you use mitt vs. min?
Why not "Det är min boken"?
You cannot have a definite after a possessor. English works the same way, since "My the book" is enitrely non-grammatical to say.
"Det är min bok" would make sense (It is my book), but your translation would be "It is my the book".
Yes, but what if it really is my the book? Give it back at once!