I think the gutter between the columns looks tight - as a result the two columns look jammed together in all that white space. Since you said this book is heavily illustrated it might be helpful to see how you intend to treat the pictures and captions as well.

And a few design comments:

1. Never start a column with the last line of a paragraph if you can avoid it.
2. Your paragraph indent is a little small - barely perceptible. Also the first paragraph under a head is usually not indented.
3. Try to align your columns if possible.
4. I might adjust the proportion of your chapter title to your chapter number. As it is now, CHAPTER 2 stands out more than “Alphabets”. It’s less important. I’d try to reverse that emphasis.
5. You have a mix of lining figs (in the text and main title) and osf (in the chapter number). I’d make those consistent - or spell out the numbers in the cn’s. Also be consistent within the body of the text about how you refer to the chapter numbers.

OK that’s all for now.

I’d echo Patty, and add:

2. . . . Also the first paragraph under a head is usually not indented.

Unless you also indent the head. But generally, a flush subhead is more serviceable; less true for chapter head, unless, as in your case, chapters don’t start a fresh page.

3. Try to align your columns if possible.

And if you can’t, one accepted conventing is that the inside columns are shorter, which would also solve the problem of not having the last line of the paragraph start a new column.

4. I might adjust the proportion of your chapter title to your chapter number. As it is now, CHAPTER 2 stands out more than “Alphabets”. It’s less important. I’d try to reverse that emphasis.

Right. Pure size is one way. Another way to lessen the impact of a larger element is to screen it. If, for example, you are using a large figure for the chapter number, and you want it to balance better with a smaller chapter title, you might screen it. The same would apply to a drop cap. Not to say you should, it is just one more tool.

5. You have a mix of lining figs (in the text and main