Do you need to keep the page count the same? What about the font size or the sizes of the graphics?
Since you're only dealing with a difference of a few millimetres, you would probably be best to take it up in the margins. All you need to do is ensure that layout adjustment is off (Enable Layout Adjustment is unticked in the Layout>Layout Adjustment dialogue), then reset the page size using File>Document Setup.
If you want the text box to expand to make use of your extra mm, make sure that Layout Adjustment is on, and all the options are checked. If you do this and there are graphics and call-out boxes, you will need to resize or move those by hand.
If you want to be super professional and adjust the margins and text box at the same time to maintain the proportions, leave Layout Adjustment on and make the necessary changes in both File>Document Setup and Layout>Margins and Columns.
You may also need to move and resize any frames, such as running headers, on master pages.
As far as I know, if you have a book file with chapters in separate documents, you need to make the changes to each document.
On the subject of margins, note that LS require a generous 13mm safety margin all round. If your book was originally printed conventionally, you may not have had to leave so much, and might have to do the margin adjustment for that reason.
Note that in CS6 Layout Adjustment has been hidden away in favor of Liquid Layout. To find Layout Adjustment you must now go to Layout > Liquid Layout and then open the Liquid Layout panel menu to get to the Layout Adjustment dialog.
My apologies, I don't have CS6.
No problem. Just trying to head off the "I can't find Layout Adjustment" problem before it happens.
Many thanks -will go away and give this a go.
Want to keep it simple so will keep page count, font size and graphics all the same. Yes, each chapter is in a separate folder.
Will use Indesign 5 if that is easier?