If you have an apple ipad, it should automatically prompt you to “open with” when you click on the attachment, and iBooks should be amongst the options. On your computer it should open with whatever your default eReading program is. Just bear in mind as mentioned below that it will appear slightly differently depending on which device/app you’re using to read it.
If that doesn’t work, you can view your eBook file by opening the file with any eBook reader capable of reading an .epub file. If you do not have an eBook reader that is compatible with an .epub file you can download a free reader from the internet called “Adobe Digital Editions” at http://www.adobe.com/products/digitaleditions. Look for and press the “Launch” button to download the reader to your computer.
The “Launch” button looks like this.