Alex I definitely applaud your efforts to write this book.
This is really great. I like how you saw a problem and attempted to fix it.
In life it is often the one who has the problem that is burdened with finding the solution. We often can't wait for others to solve our problems for us. I like that you shared your concern. Often times when we share we find people more able than ourselves who are willing to work towards a solution.
I really do applaud you for working on this book, I just think its awesome. Often times people complain about something but do nothing, you my friend are taking action. We need more people like you my friend.
Now my suggestion is that you start basic then refine.
Instead of just jumping into the book. I suggest going back over your outline.
Get it really fine tuned and even lay out sub headings to give you a blueprint of what the layout will be.
If you don't do that I fear you will end up trying to connect the dots as you go along or force dots to connect to which may lead you slightly off topic. I think a solid outline is very very very crucial. There is no need to set sails on the grandest ship of all if you have an incorrect map of the sea, the journey will be doomed before it starts.
Someone made a point about the topic of the book. I agree that you should not make the book too specific. Determine your audience first. My assumption is that you are not making the book for yourself but for others. So ask yourself am I writing this book for:
(a)beginners with basic C++ experience but not allegro experience.
(b)beginners with no C++ experience and no allegro experience.
Based on if you select A or B above, the book will be different. The main difference will be that for those with no C++ experience you would have to teach them C++ first before you can teach them about the Allegro library.
My recommendation is that you refer those that don't know C++ to a C++ tutorial to read first and just focus the book mostly on A5.
I think you should aim to teach basic concepts and you can be specific as in teaching specific algorithms or programming design modals.
For example, you said you do web programming. Something common in web design is login features. That is something one could teach in a web tutorial. With games something common may be a side scrolling game, you could teach how to create functions and classes to handle player movement. The danger though with being too specific is that you are assuming the reader wants to duplicate what you are doing.
Instead of teaching someone how to make a car game, it is better to teach them how to create an object, how to control objects, how to display objects to the screen, and how to create artificial intelligence for objects. With that knowledge the person can take those tools and make any game, a car, plane, boat, shooting, etc(just an example).
Yes for beginners I do think it is valuable to include for example a car game source code example to show how the components fit together, but don't make that car game(for example) the main point you are trying to teach them, make the components that make the game possible what the focus of the book will be. That way you foster creative thinking in the person and they can take the components and use them independent of the example in the book.
Once again good luck, I'm available to help with your book, email me at firstname.lastname@example.org, I can help out on the outline and some theory and concepts and some code.