Dear Mr. Wallace,

　　I am writing to invite you for giving a lecture about "American literature".

　　Since more and more students have shown their interest in the object(subject) about American literature, we() the Students' Literature Organization is going to hold a lecture to follow this trend. It can be conspicuously recognize(recognized)that you are an excellent expert in the field of literatures all over the world. What we want is to(we sincerely) invite you to give a specific speech on this content. If you are convenient on next Friday afternoon, we will welcome you to our lecture(you will be warmly welcomed).

　　We would appreciate it if you would give us a reply as early as possible.　　

Yours sincerely,

　　Li ming

　　评：本篇文章在有效的表达了邀请的意思，原因阐述比较详细，但不足的是语法，句式，词语方面略显不足，须在这个方面有所改进。