How to run iBooks Author app in Snow Leopard
To run iBook author in Snow Leopard, first download the app from App Store. You have to temporarily change you system version number to 10.7.2 in order to download app.
First go to HD:/System/Library/CoreServices/SystemVersion.plist backup & open it with text-mate and change the string value of 'ProductVersion' and 'ProductUserVisibleVersion' to '10.7.2'. Save & Overwrite.
Download the app from Appstore. Restore your backed up SystemVersion.plist. Now right click on the download app & go to Show package Content/Contents/Info.plist. Open it with text-mate and find the key 'LSMinimumSystemVersion'. change it's string with your original system version for example: '10.6.8'