These are the two top books that have the best information.
*** I consider them both to be absolutely bottom-line basic resources to have if you are intending to understand the topic of low carb nutrition or nutritional ketosis. ***
These can be ordered in paperback.
They can also both be obtained as Kindle books, and for very reasonable prices (currently $10 and $6).
Note: you do not at all have to own a Kindle device to read these books in Kindle format – you can download the Kindle program or app onto your desk computer, your laptop, or your tablet or phone.
For convenience, here are links. I do not have any financial relationship with the authors or with Amazon, so I get nothing financially whether you use these links or not. I do not find them listed on the iBooks store.
The Art and Science of Low Carbohydrate Living
The Art and Science of Low Carbohydrate Performance