At the first glance it seems this is written from your own experience and perspective rather than as an overall game design guide. While it gives a basic outline of things it seems to focus heavily on programming and largely exclude the actual design part of game development. How to write a good story, how to make appealing graphic content, how to understand and use music, how to have interesting gameplay and intuitive controlling schemes? Etc.
For getting a good grasp on game structure and game development process I feel it could seriously use more graphs, diagrams, concept maps or other forms of data that is quick and easy to overview and process. But I understand all of that takes time which you might not feel comfortable spending now that you're publishing it for free.
Still, couple of quick things to improve on:
1) Add some art and graphics to break up blank text pages. You're publishing digital media, ink and color do not cost you. Many digital books even have white-on-black basis to help with on-screen reading and it also presents art nicely. You can also consider finding a novice graphic designer to help you improve on the visuals. It's not all just aesthetics but good visualization helps you make your points.
2) Check your topic hierarchy is logical and clear. For example the things you list under "contributors" subtitle is confusing. And do you really need to write a chapter about the gender of a game developer when your message is it doesn't matter? I think by including a chapter like that you are sort of answering a question nobody is asking and by doing so actually assuming it would matter.
Also the intended topic hierarchy is currently a bit hard to read both in table of contents and the written part. So while you might adjust your hierarchy, you could use visualizing that better as well. You could indent the chapters and topics more, use different font formatting (some color shades perhaps) in different level titles and text.
3) Check spelling and grammar but mostly try to improve on sentence structure and variance. Also check your big scale chapter structure is logical.
If you don't want to polish on this yourself you could search for an editor for your book that might be able to point you better direction in many aspects. For the right demographic it could be a book that practically reads itself but at this phase it looks like something you wrote chapter at a time on a subject on your mind and just assembled it all together.
Still, everyone can see you've put a lot of effort into it and I'm sure there's lots and lots about really useful programming information in there. For this you obviously deserve a big thanks. But also you'll need good structure in your large document so that people can find and understand your points and benefit from your knowledge. 
