Can you suggest information sources about tax rules about annual retirement savings limits and deductions applicable to married couples filing jointly by income? Tx.

One spouse has employer 401K match and the other one does not. Interested in how to maximize that. One has existing pretax and post tax accounts. Would like to better understand rules about what can or cannot be pre tax and post tax  going forward. Links  would be great. Tried IRS but wasn't getting a good document match. Thanks.