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If I were the author in Stefan's situation (not getting much money from the book anyhow), I would simply want to see my work being used and appreciated. As you said, it's a lot of hard work; I would hate to see it left abandoned by my audience.

That said, if the promotional period is a success ...

It sounds like you had a very sad publishing experience, Stefan :( I've never heard of a publisher wanting rasterized graphics, but I suppose that their printer itself might have some…odd requirements. Luckily, ImageMagick comes to the rescue :)

I mainly wanted to comment on your experience, but I ...

(Edit: I misread. The following will keep *alignment* from line to line, but it will not be centered. You may be able to combine \intertext with some other equation environment, but I haven't the time to experiment.)

Use \intertext from the amsmath package:

\documentclass[12pt,letterpaper]{book ...