On the topic of GM Screens, don't forget that there are two sides. Filling the side facing the GM with useful information is good. Fill the side facing the Players with something useful as well. Personally, I used to keep a library of cool background images that I could switch out depending on the scene the players were in. Admittedly, this is easier when the potential backgrounds don't include the black of space, but I've found it can help set the mood. And if you have pictures of your NPCs, that can be good to flip over when the PCs are talking with him/her/it.
Of course, screens probably should be on the website, not the book, unless you are making some very fancy screens.
Regarding what to cover, I would also recommend thinking about how Icar is different from other games. If something is bread and butter for all games in the genre, you can afford to spend less time on it. But if something is unique (or nearly unique) to your game, more time is necessary in order to help GMs properly frame the concept.
Just a screened,
Thought