The book and inventory tabs switched places between the two images, is that meant to happen? I think they should stay in their place when selected, and just be highlighted.
1. Font readability: I think serifs are a poor choice for a pixel font. This font has some spacing issues overall, too.
2. Tab spacing: I think it would be better to align them, yes. I'd also have less visible thickness to the pages, which would make them being aligned look better, and give you a little more space to work with. You could really use that space too, some of your text gets too close to the page edge/spine.
3. I think it's distinct enough. Like I suggested earlier though, you could add more non-functional background tabs behind the menu ones just to replace the negative space around them, thus more clearly separating it from the gameplay area.
4. I think this is a fine amount of text, it's more than I'd want to read all at once before getting to make a choice or take some other action.
5. The line spacing is fine, but the font itself isn't very readable because there's no spacing between many of the letters. Also, try moving it down a pixel, so that the bottoms of the letters align with the lines. Currently, they sort of blend in with the lines, making the bottom serifs look really thick. Here's what it looks like moved down a pixel, I think it looks cleaner:
It might look better or worse with a sans-serif font though, so try it out.