You do not need to submit a job offer letter because you are going to apply as his derivative. You need to submit proof of your status, proof of your relationship to your spouse, medical exam, proof of bona fide marriage (if you had married recently USCIS will require you to show that your marriage is bona fide even though you are applying through employment), and all the documents listed on the application form. It will be best to consult an attorney to make sure that everything is properly documented. Also, maintain your H-1B status until you receive your permanent residence, if you can. Priority dates progress and retrogress, so if your adjustment is rejected for some reason, and the priority date retrogresses, you need to have a valid nonimmigrant status to avoid accruing unlawful presence.