I have been designing, troubleshooting and commissioning process plants since 1991. This is the area I write about. 

I write the first draft of my books quickly, straight off the top of my head, based on my personal experience. I then have the draft reviewed in parts by large numbers of other practicing engineers to ensure that it is complete, current and correct. 

I then review the reviews, and the sources they suggest, amend and add to the text and then start to bring the text together into a unified whole. Repeated review, rewriting, refining and proof reading takes up far more time than the initial writing stage. 

My books are therefore based largely on my own experience and that of the hundreds of other practising engineers who contribute to and review the texts. They are intended to assist others with the process of making engineering education fit for purpose, and to show those wishing to become engineers what practicing engineers think engineering is really about.