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Watertiger - in Viewer preferences (gears icon) there's a setting for number of pages, I think its in the Page Layout section
Re the mouse scroll jerkiness - wouldn't have thought it would be the Magic Mouse. What happens if you hold the down the
↓ arrow key in Flow Mode?
IIRC the viewer reads an unpacked instance of the book from the temporary storage - what sort of device is being using for temporary storage.
If your books are ePubs: an ePub often consists of more than one file of text - typically one file per chapter, where a slight delay is hardly noticeable. Maybe your books consist of a large number of xhtml slices. Simplest way to check is to open the ePub in the editor.
If your books are not ePub, e.g. they're MOBI: then the viewer does a background conversion to ePUB when it opens the book. Try doing a regular conversion to ePub using the default settings and viewing the resultant ePUB. If that's jerky have a look at it the editor, if its not jerky then...
BR