I got to know that it was a self-help book.
This means that you learned in the past that the book was a self-help book. Use this tense if you are writing a paragraph in the past tense. If you are using present tense in your writing, with this line in past tense, it could mean that the book has changed since that point in time.
I got to know that it is a self-help book.
This is present tense and means that the book is currently a self-help book.
I got to know is an unusual phrase in describing an acquired bit of knowledge. I would recommend instead, I learned, I found out, or simply, I know.
I found out that it is a self-help book.
I learned that it is a self-help book.