Another Beginner's Grammar Question

[improperlyhuman]

I'm totally new to Russian grammar, so I want to know if my thinking process is incorrect.

My sentence is

Вечером я пишу в своей гостиной.

I guessed from the ending of гостиная that it is feminine, and в is a preposition, although not expressing motion. So, I looked at the endings for singular feminine nouns in the prepositional case. -ой is not listed on my chart. I also have Schaum's Russian Grammar, and it's not there either. I don't understand this noun suffix.

Thanks