How much can I make Writing a BookWhat can I do when I write a book?
While you' re already a celebrity, you can make a great deal of it. You will probably find that if you are not already renowned, you would write a less tech book per lesson than you would write a notion, and you will write less than you would just work as an engineering in the class.
I' d say that the Java book area is overcrowded. When you look at Amazon, 100 "best-selling Java books" are listed; there is nothing more because they score at 100. Likewise PHP has 100 such ledgers. It' similarly crowded, but I would say that a PHP "bestseller" probably sold much less than a Java "bestseller".
Find that you probably speak for a tech book for at least 2,000 of your own work. This is a full-timers' position, 8 hrs a year for 50 week a year, after that you get 2 week holiday for good manners. Compar this to a female writer like Danielle Steel, who can publish several novels a year (she usually juggles 5 different works at the same book, each one lasts about 2.5 years, but much of it is "wind it up and let it go", where she doesn't really bother to publish the book.
Asimov was often referred to as "his own book of the months club". So, if you're going to go on volumes and assume you're moving 20,000 units at a linked cost of $24, at a 15% toll (which is generous), then move about $72,000/year for your tech book, but $144,000 if you're Danielle Stahl, and $864,000 if you're Isaac Asimov or Stephen King.
Even if you are writing the Canon Law Book on a topic like PHP - unless you are a university lecturer and compel your student to buy your book as their text book (a popular strategy), you will not be universally known. Specialist literature is usually a bestseller if it is the only book on a topic.
However, even "Go" currently has ten titles on Amazon. So, even if you are thinking of something obscured so that your book can be the book.... you are unlikely to find a topic where that will be the case. Then you can sell your book to your trapped people.
However, you become known more as a talker and end up in the discussion and advisory circle than getting wealthy through a book.