First off, is it a digital book or hardcopy?

Now, a few questions to ask yourself.

How do you hope to leverage this book? Is it a means to grow your reach (e.g. email subscibers for your blog or promotional)? Or are you looking for cash? Books are most often used as a way for you to position yourself as a thought leader in a space so that people will seek you out and pay you for your expertise. From this, you can aquire notoriety, enough to potentially get sponsorship deals or endorsements and bring on clients as a consultant. This could look like 1:1 work, courses or seminars.

The number one question you need to answer is how you intend to leverage what you hope to create with this book. It's not a matter if it's ok or not to give it away for free, but rather, does it serve your goal? And what is that goal?

Hope this helps and happy to clarify further if you have any questions.