iPad makes reading books and other publications convenient and easy. You can purchase electronic books from the iBooks Store and other bookstore apps. And, you can download books - thousands upon thousands of them - from the web and install and read on your iPad in the iBooks app.
First, a little tech-speak.
Electronic books are commonly created in ePub and PDF formats. You're probably already familiar with PDF files. You typically view them in Adobe Acrobat Reader or the Mac OS X Preview application. The ePub format was created especially for electronic publications - like electronic books and magazines. While PDF files display a book as it was originally formatted, content in ePub books is reflowable, meaning that things like text size and text font can be changed for the best reading experience on a particular display device.
Many publishers offer their books in ePub and PDF formats. For example, I've been reading the book "MAKE: Electronics" from O'Reilly Books - a wonderful primer for aspiring electronics geeks.* Instead of purchasing the hard-copy book, I downloaded the electronic version from the O'Reilly Books web site. And, because they offered the book in both ePub and PDF formats, I was able to bring it into my iBooks Library on my iPad.
Here's how to get electronic books you download from the web into iBooks. It's easy.
1) Download a book.
Download the ePub or PDF version of a book to your Mac. Book web sites typically tell you what format a book is in.
2) Drag the book into iTunes.
Launch iTunes and then click the Books library in the left column (in iTunes 9.1 or later). Drag the book into the iTunes window to add it to your library. (If you don't see the Books library, open iTunes Preferences. In the General tab, click the "Books" checkbox to enable it.)
3) Sync your iPad.
Sync and your book will be added to your iBooks library. Books purchased from the iBooks Store and ePub books are displayed together on the same library shelves. PDF books are displayed on their own library shelves.
Find Books on the Web
Here are some places on the web to find electronic books: oreilly.com, ebooks.com, gutenburg.org, fictionwise.com and books.google.com.
*By the way, O'Reilly Books offers lots of wonderful Mac-related books, such as "iPad: The Missing Manual" and "Mac OS X Snow Leopard: The Missing Manual."