Writing technical books sharpens all of your writing skills. Your ability to plan projects improves. Most authors notice that their writing speed increases dramatically the further they get into a book, usually to between 10-15 finished pages a day in the last stretch.
Writing technical books can be profitable. Furthermore, royalties are “found” income. You can expect royalties for several years after the book is published in addition to your current salary. Publishing a book also looks very good on the resume and impresses clients greatly.
Most importantly, writing technical books is fun. Although I like to joke that writing books is like hitting yourself in the head with a hammer (“Because it feels so good when you stop!”), it’s been an incredibly valuable experience for me. When I got my promo copies from the publisher, I had a tremendous sense of satisfaction. Seeing my first book for sale in the bookstores was one of the great joys of my life, and it will likely be one of yours, too.