My husband died and was part of a closely held corporation that did NOT have a shareholders agreement… what documents are needed from the company to proceed?

1) You need something showing how many shares of stock of the company there were, and how many your late husband had--that will determine his ownership interest. E.g. if 300 share were issued and your husband owned 100, he was a 1/3 owner. The other two should be able to tell you these things.
2) Valuing a closely held company is very difficult. If it manufactures things, purchases and distributes items from other companies, or owns real estate, you can approximate its value by the value of its inventory, real estate holdings, equipment & vehicles, etc. --but that is only a rough approximation. If it is primarily a service business, the value is probably between 1 & 2, maybe 2.5, times the annual gross revenue. (Again, that is only a rough and general approximation.) So getting information about inventory, after assets, and about gross revenue, will help you get a sense of the business's potential value.
3) Since you can't sell a closely held corporation as a minority (owning less than 50.1%) shareholder, and there is likely no real market for anyone to buy your share of the company (since anyone who bought in would be "at the mercy" of the brother- and sister-in-law who, we presume, together own more than half the business and can therefore make all decisions without regard to what the third owner wants), your only real option is to discuss matters with your in-laws and decided on some mutually agreeable amount for them to buy your interest out for. Getting back to 2) above: say it's service business with annual billings of $300k. That makes your late husband's share (assuming he was a 1/3 owner) worth probably $100k - $200k. Use $150k as an average for this example; apply a discount to reflect that you are selling to family, but doing so without having to hire a busines broker or the like, and so saving money. You might agree to sell the in-laws your inherited interest in a business worth $150k for $100k - $125, possibly payable over 3 - 5 years in installments.