Is a patient's entering to see a doctor without a prior appointment, under the pretext of giving him a book for his daughter and in response to his suggestion, considered an injustice to those who had appointments before her?

If the custom in medical clinics dictates the right of whoever booked an appointment, then it is not permissible for someone who arrives late to precede those who arrived before them unless they give permission. What you are doing by resorting to deception does not change the reality of the matter, which is an injustice that is not permissible. We advise you to ascertain the doctor's free times, inquire about the number of present patients before going, or book an appointment in advance by phone, or delegate someone to book for you.