Here is a process for getting quick feedback to write a compelling book that readers want to buy.
Straw-Man version is the first draft. The quickest way to create the first version is to have your friend ask you questions about the topic and you answer them. You can record this session as a video.
Define the scope of your book by stating what you will cover and what you will not cover. Use constraints to your advantage, for instance : This book will be about 100 pages. I will spend a weekend to create the Straw-Man version.
It is also a good idea to list top 3 problems that you will address in your book. Practical, hands-on approach to the book that helps the readers to accomplish their primary goal will be easy to sell and spread by word of mouth.
For programming books, you can come up with some examples you will use to illustrate the concepts. The first example will obviously be an hello world example.
Get the video transcribed on Elance or Odesk. Revise the transcript and make it flow smooth by arranging the sequence of concepts. Work your way backwards and create a table of contents for the draft version. To create table of contents you have to group paragraphs into sections and sections into chapters.
Use the Straw-Man version material to teach a small group of people (2-3). This will create what Napolean Hill calls as the Master Mind. Record the teaching session. Students will ask questions on things that is not clear. Answer them. Also observe your students and see if they are able to accomplish the learning objectives. It is important that your material helps them. Because your promise in the beginning of the book must be fulfilled if you want to be one of the best selling books.
After the class, revise your material again based on the feedback. This will result in Wood-Man version of the book.
You can have a group of 5 to 10 to teach a class and repeat the process as before.
Take the previous version of your book and make a presentation to an audience of 50 to 100 people. You will have to answer people at various skill levels which will be a challenge. After revising the material you will end up with the Iron-Man version.
Your book eventually becomes a proxy for you. It is able to handle more students as you progress through the different levels.
Remember that Iron-Man has his weakness. This version does not mean your book is perfect. Aiming for perfection will not help you publish the first version. You can always refine your book later based on reader feedback.