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Another trick, if you don't have anything flat to align and you're having trouble figuring out the correct rotation: align the screening.

The screening, for any particular ink color (CMYK), is at a particular angle. This is probably the same on both pages. Use the ruler tool, and align it against a particular line of dots. Try to pick an area where a single color is clearly visible (different colors are often aligned to different angles), and where you have a long, clear shot--the longer the ruler is, the less error. Note the angle in the info pane. Do the same thing to the same color in the other image, then subtract them, and rotate one of them by the result (or the inverse, depending which way you're going).

The info pane, unfortunately, only shows one tenth of a degree, when an exact alignment could really use a hundredth, but it can get you pretty close. Once you have the correct angle, also getting the position aligned often becomes much easier. This only works on unfiltered raws, of course, and it won't work on high-resolution FM screened printing.

(Actually--more advanced: you can get a number to the hundredth by opening rotate arbitrary; it'll be modified, since it's trying to snap to an axis, but the same subtraction will still probably work. But you'll need to yank the parts back into separate images and use rotate arbitrary to apply a rotation with that precision, since Free Transform seems to round to .1 degree.)