I would say that if you can commit to 1 hr per day for 2 years, or some other combination of effort, a book is do-able. In truth, that is too much for those with just a casual desire to write a book. If it is, maybe consider a short form writing format instead, such as a blog.

The most difficult part I found was mastering the documentation system and automation. This is important if you want to deliver to multiple target formats. Your book needs its own Continuous Integration system. The effort here pays for itself many times over. For concrete details, I documented my journey at https://github.com/faisalmemon/ios-crash-dump-analysis-book

The next most difficult (but fun) part of it is the primary research. Most of what you say in a book is known knowledge but just compressed down from upstream reading material into the story you want to convey. The extra piece is your own insight. To have a "solid" book, that piece must be rooted in some primary research (so you know what you are saying is "correct").

The actual "writing" part is the easy part! The main problems are stylistic errors but you can search the internet for tips to avoid the pitfalls. (e.g. avoiding repetition, being concise, avoiding complex words when a simple word fits better, etc.)

For spelling and grammar, I just exported to MS Doc format, and used its in-built checker. For proof reading, I just used the Mac text-to-speech facility to catch mistakes.

I was quite fearful of the response given my amateur approach but experienced quite the opposite. My book was well received. (Amazon 5 stars!).