Hi Lee, Great question! I have tried to explain the sentence below. I have just bought a book. It has some common mistakes in it. The book was probably written in a rush. The quality of the writing is poor. It is shoddy work. I noticed the mistakes. It is easy for me to see the difference between the quality of the writing in my book and in other truly excellent ones. Truly excellent work does not have common mistakes in it. Common mistakes separate some books from other, well-written ones. I hope this answer is helpful to you. Happy studying!

Read it like this: (Common mistakes like these) will separate (rushed, shoddy work) from (the truly excellent) - or to paraphrase: "these mistakes separate bad work from good" In other words - those common mistakes will be the difference between the good work and the bad work. BUT - I don't think it is clear whether they are saying that making those mistakes will make excellent work bad, or whether seeing those mistakes will allow someone to easily see which work is good or bad.

Your interpretation doesn't make much sense, I'm afraid. It means that work that contains "common mistakes" can be distinguished as "rushed and shoddy" compared to the "truly excellent" work.