I wrote a book, too (career advice for new devs). I had previously written a technical ebook (about Cordova) and wanted to write something that had a longer lifespan, but was still focused on developers. Writing a technical book was great, but, like you said, it became outdated very easily.
That said, if you want to anchor yourself as an expert in a space, I can't think of a better way to do it than to write a book. This is because it will force you to learn the topic very well (I remember spending an hour testing out something so I could write one sentence accurately) and because you can say "oh yes, I wrote a book on that" which gives you a lot of credibility.
I think it was totally worth writing the less technical book too. It makes you dig into thoughts and ideas in a much deeper way than blog posts or presentations.
It also opens up some doors in terms of presentations and speaking, if that's of interest. My book wasn't on a technical topic, but if you write one, I could definitely see that leading to some consulting.
I think the best way to write a book is to blog it first, then assemble the pieces (adding in more). This lets you do a number of things:
* you can outline the topic as a series of posts. If you can't do this, don't write a non-fiction book.
* you can see if you like the topic 5-10 posts in. If you do, then you have the bones of a book. If you don't, why would you commit to a full length book?
* you can build an audience. I didn't have a large audience, but had a list of 200ish people I could market the book to. And again, if you blog 5-10 times and no one is interested, they probably won't be interested in a book either.
* you can easily turn it into an ebook (using a tool like leanpub) if you determine you want to self publish. If you work with a traditional publisher, you can still pull and revise the blog posts.
Since the book was just released in August, I have no idea of the financial success. Like the author, I'm not looking to have this be a huge piece of my income stream. If I can earn out my advance, I'll be happy.
Here's the book URL if you want to check it out: https://letterstoanewdeveloper.com/the-book/