Things you should know.

a 1,000 book, but cocked !
second photo (with top edge dusted) taken just 22 mins after the first:

Strictly Kitchen:

How to straighten Skewed or Cocked books: the Bob Jackson method*

[nb: Later comment: I now omit steps i & ii: whatever works for you.]

brush cavity
i: To give access to the spine cavity, open the book wide and stand it up.

ii: Damp the spine of the text block: I use a soft brush poked carefully half way down the hole from the top: then invert the book and repeat.

iii: Close the book and skew it back, slightly beyond straight, to an opposite skew: secure it in this position with an elastic band vertically round the boards, angled, where it crosses at top and bottom, so as to maintain the pull.

iv: Microwave the book!  I use full heat: my experience so far suggests that this should be done until the spine has become very warm or quite hot, but then stopped. How long this takes varies with the book. Perhaps, with a small book, check after 10 seconds, and then at 5 second intervals. [That's pretty conservative - many books take over 30 secs]

v: Leave the book for a while to cool off before removing the bracing. It may retain its shape after sitting for 10 mins, but keeping it held in position for longer may be better for a permanent result. [Definitely worth leaving for longer.]

Obvious or not:

This method can work only with books small enough not to catch on the sides as the table turns.
Check there's no grease or other gunk on the turntable.
Experiment with cheap books first ! - and stay vigilant. Staples spark. Some kinds of paper heat up fast and could catch fire, and other materials may behave strangely.
If you think there may be ill effects to the cloth spine or its lining due to steam from the back of the text block, you could slip a strip of polythene down the spine cavity as a barrier. 
Non-glossy paperbacks (such as early penguins) can be treated by damping the printed spine.
Microwaving without damping works sometimes - perhaps when the books have retained some humidity.

Persons attempting these procedures do so entirely at their own risk.
I take no responsibility for their effectiveness or safety, and accept no liability for any damage or loss incurred.

*I first envisaged and tried this procedure on 8th July 2011. I hope you find it useful. Feedback welcomed !

I have subsequently found these related suggestions online :
i:  a very old (2001?) online posting by one 'Blake at LDC' for gently microwaving paperbacks "to warm the glue inside the spine" to "correct off kilter or rolled spines" - but with no mention of damping or bracing them, or of any procedure for hardbacks. Google the phrases quoted to find the extensive compendium of tips on book collecting & care from which this came.
ii: in a 2007 thread on about correcting skew, the discussion rambles around ways of applying sustained pressure for a long time (weeks, months, a year or more are mentioned) while a new geometry is enforced, and one J Godsey adds: "if it seriously cocked, microwave it for 10 seconds before you reshape it."
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