Before ordering your book preview it using this method - http://support.apple.com/kb/HT1040 - and save the resulting PDF for reference - the delivered book will match it.
The resulting PDF is quiet large sice it is set for high quality printing
Actually, I have found an even better way to share online. I too have been in the habit of saving my books out to PDFs and when those became too large, even for YouSendIt and the like, I ran into the same problem you did.
I have discovered that if you click on your book in iPhoto and then select "Export" under "file" that you have a number of options to choose from. The best I feel is the "Slideshow" option which allows you to create different types of video formats. I for example use the export to .mov and create a fantastic QT file that plays the book out exactly as it is. The other options do not seem to do that for some reason.
The only part I have yet to figure out is how to control the speed of the page turning (yes it creates a nice page turning effect for the book). Of course if one wants to read the text in the book one has to press pause on their playback.
I then have uploaded those video files to YouTube, Facebook, etc. and shared them from there.