回複：Legal brief?

How many published papers do you have? More than 20? Do you have strong recommendation letters? If your background is very strong, may be you file I-140 by yourself is OK. If not so strong, you may not able to prepare by your self. I think you'd better to ask a cover letter sample from your friend(s) who have got I-140 approved; because the cover letter is very important. Especially, if you file NIW, you need to prepare a very professional cover letter to cover all of the key points required for NIW petition.

God bless you, good luck.