I have a couple of questions i would be grateful to ALLAH if they can be resolved:

1) What are the rules that Islam sets for sex
2) What are the times that are forbidden or makruh for sex
3) Can we have sex twice or thrice a day?
4) Is it harmful to do it every day
5) Using protection for a longer period to avoid pregnancy is not haram right?

Can you share any books?

The times that are haraam for sex are during the woman's menstrual cycle and when one is observing a wajib fast. Other than that it's not haraam. In regards to makrooh times, it is said the first, middle and end of the lunar calendar are makrooh. From a Islamic laws perspective, having sex 2 or 3 times is not haraam but in regards to your question about it being harmful, that is something for science to answer and also depends on both the husband and wife. Using protection is not haraam.

Below are two books you can refer to for in-depth information: