If by sth you mean the word ‘something’ the correct sentence would be -> ‘develop an interest in something’. Assuming ‘something’ is correct, the sentence is more like If you tried reading a book for the first time and you really enjoyed it then you started read more and more books because you enjoyed it. In general it’s when something interests you and you try to learn more about it, it’s not necessarily instant. You could try something and hate it haha.

To develop an interest in something is to become interested in something gradually. It’s not an instant thing like falling in love with something. Example: “My brother always watches basketball on TV. I never liked it before but now that I keep seeing it everyday, I’m developing an interest in it.