I definitely agree with the idea of writing books and publishing courses.

With the advent of publish on demand (Createspace/Lulu etc.) and ebooks (Kindle/Kobo/iBooks etc.) becoming an author is certainly a possibility. There is money to be made in this area, however, most IS/IT consultants will make more elsewhere. The trick is in using the book to drive new business. Send a copy of your book to prospective customers. Give it to newspapers and radio stations. It can turn you into a percieved expert and help build your business. (Be warned there are downsides to the strategy).

Recording courses for web distribution or even as a physical product also work well. Sadly they actually can be sold for more than a book can. And the effort is roughly the same. (Recording a course is easier than most people think).

Just don't jump into the business without learning it first.

Glen Ford
However, you do need to remember that both these strategies are businesses and need to be treated as businesses. Even if they don't pay for themselves directly.