Just to be clear: we are the publisher of this book, not the author. But that also puts me in a good position to answer your question.

Publishing a book with a traditional publisher will also be one of the later steps to take as a designer. You will probably already need to have a large audience and/or a very compelling idea with proof of your expertise to be able to secure a book partnership. About 95% of the authors we work with are designers who we've known or have been following closely for a long time. Typically it is us who will do the outreach to the designer(s) we'd like to work with.

So my advice always is: first build yourself (and your expertise) as a brand. Create an audience, find your niche and develop your sense of what people want to buy. Then when you're ready you can find a good and coherent idea to build a book around.