By itself, it’s unclear. It could mean, for example, ‘I’m working on a book.’ , ‘The book is in the process of being published.’, or ‘The book is in print.’
The phrase ‘in production’ (generally not ‘under’) is often used for a new product that has been designed but not yet on the market, or a movie that is in the process of being made. With books it’s more common to talk about ‘publishing’ or ‘printing’ than ‘production’.