You don't need to jailbreak your iPad to read DRM-free ePub books:
How to transfer ePub ebook file onto Apple iPad, iPhone, iPod Touch

Here are some legal source for free ebooks (including public domain ones)
Where to download free ePub eBooks for Nook, Sony Reader, iPad,...

If you have an ebook format other than ePub, you can try to convert it to ePub using freeware Calibre.