我能理解你想表达的意思,但是原句不太通顺,还有语法错误啊.我想这样说比较好：
An e-book is the electronic version of a book; the difference between the two is the same as that between a letter(mail) and an e-mail.
翻译：电子书是（纸质）书的电子形式,二者的区别就像（纸质）信件和电子邮件的区别一样.
希望对你有用.