The Simple Present of the verb to have is slightly irregular, since the bare infinitive is have, whereas the form of the verb used in the third person singular is has. (she has, he has, it has) do , does (for the third person singular) are auxiliaries to form questions and negative statements in the Simple Present. When the auxiliaries do, does are combined with another verb, the other verb always has the form of the bare infinitive. So, she doesn't have a book.

now we move to the negative statements : we say : she does have a book the negation : she does not have a book the short form is : she doesn't have a book

we say : she has a book the tense in this sentence is the present simple . And the sentence is formed unconsciously in the human brain like : she does have a book the auxiliary takes the tense of the main verb " have ". And the process of making questions in English is based on subject-verb inversion ; so : Does she have a book ?

You can think of it in terms of which verb is conjugated to match your subject. Here are examples. #1 [Subject]+[conjugated verb] [article] [object]. She has a book. He eats a cake. They ride a rollercoaster. #2 [Subject]+[conjugated verb] [negation word] [object]. She hasn't a book. He eats no cake. They ride no rollercoasters. #3 [Subject]+[conjugated verb] not [infinitive form of verb] [article] [object]. She does not have a book. He does not eat cake. They do not ride rollercoasters. Note that example #2 has a slightly different meaning than #3. #2 emphasizes the verb as the thing that is not done, rather than the object. (He eats no cake = he might be a cake decorator but he eats no cake. He does not eat cake = he eats cookies and chocolate but he does not eat cake specifically.)

Thanks a lot. Your explanation is simple and yet very easy to understand.