Suggestions for restoring backyard vegetable garden.

Zone 8b, partial shade

I have been offered some space to garden in my in-law's yard. It's been sorely neglected and needs some restoration work before it will flourish. What would you suggest I do between now and Spring to nourish and amend the soil, and what crop suggestions do you have for next year?

Some background; this space was set up with raised beds approximately 30 years ago. Over the past 15 years the beds have been left fallow, unamended, and unweeded. Two years ago the raised beds were torn down (by brother in-law) and the soil was leveled out and roto-tilled; no compost or manure was added, and no weed control steps were taken. Some seeds were planted, but the results were unsatisfactory. This past year the space has laid fallow, completely untouched. Now it is the end of the year, first frost has arrived, but the ground is not yet frozen solid. I have been asked to prepare the soil and plant it next year - with the expectation that if all goes well I will continue to have use of this plot for as long as I continue to steward the space.

My questions are:
Is it too late to add compost and/or manure to add nutrients?
What additional steps would you recommend before spring planting?
What types of crops would you suggest for next year's plantings (I was thinking three sisters, but am concerned that it may be too shady)?

It does depend how much time and effort you have to invest in it. Double-digging is the traditional way of preparing ground in winter for spring planting in the UK. It burys all the surface plants so they are killed and break down in the soil, feeding it. The earlier you do it the better, because the winter conditions help improve the soil texture for spring planting.

If you are comfortable with using barrier products (many people here are not) then covering beds with thick black weed barrier plastic can be brilliant. It kills off your weeds, needs no digging, and helps warm the soil earlier in the spring. When you are ready to plant you roll it back, rake the soil surface and get planting. If you have good quality plastic you can store it through the growing season and resuse it for many years. Do NOT use the mesh stuff - it lets water through, but inevitably shreds down to plastic fibres which you will never remove from your soil.

Lastly you can use a thick THICK layer of mulch. Cardboard first, followed by the mulch. I've had mixed results in my climate. It does a reasonable job of suppressing weeds (some spot weeding needed) but eventually turned into a slug paradise so I have gone away from it.

Enviable situation.  I used to farm peoples' back yards.  You're better off than having to remove ornamental plantscape &  landscape fabric or plastic, wait out degradation of chemicals.  Swales to distribute roof runoff, light-moderate compost till, winter-spring cover &/or crops, moderate mulch, starts in spring, then heavy mulch to supress weeds/ hold moisture/ buffer temps/ feed microbes among anything that volunteers/ comes up/ you've planted.  Legumes over manure.  Garlic!

Is it too late to add compost and/or manure to add nutrients?v

I don't think it ever is too late to add compost.

Get that organic matter into or onto the ground and let it get to work is my go-to. If you want to utilize manure I'd recommend only using aged/composted manure if you want to get plants going next year. You might want to add some finished compost in spring additionally depending on your soil conditions. Soils that are closer to mostly sand or mostly clay require more 'work' than loamy soils.

I really enjoy biodegradable smother layers. Clean cardboard or kraft paper topped with a few inches of compost topped with a few more inches of mulch may have you set for next spring. The smother layer will help mitigate the weed seeds that exist but you might get some pop ups. I have found weeding in this system to be pleasant as the soil tends to be loose by spring thaw in my zone.

What additional steps would you recommend before spring planting?

If you can, try to get an idea of what the soil actually looks like. It being a garden in the past that has laid fallow might indicate that you still have more fertility in that spot than the rest of the lawn. You might find that it only needs a little cleanup before you are getting great results depending on what were causing your issues the prior gardening season (Weed pressure)? From there, you can work towards amending but I find compost as a widely-used cure all for issues. The better the quality of compost, the better the end product.

What types of crops would you suggest for next year's plantings (I was thinking three sisters, but am concerned that it may be too shady)?

It is hard for me to give specific recommendations without seeing the site and musing on it but I'd recommend a nice crop of legumes to be incorporated with whatever your growing. Building up some nitrogen fixating bacteria in the soil along with nitrogen is a great start plus an edible yield. No matter what you do, having roots in the ground will get you going on the right track to abundance.

I like the advice you have been given by Timothy.  That is exactly what I was going to suggest.

I want to add that now is also a good time to start a compost pile.  An unused corner might work great.

Put veggie scraps, leaves, grass clipping, wood chips, etc. then wait for the magic to happen.

Here are a couple of posts to get you started:

https://permies.com/t/4624/composting/Composting

https://permies.com/t/145646/composting/Tips-Home-Organic-Compost

Along with feeding and prepping the ground, think about how you're going to manage your local garden bandits. It can be horribly upsetting the first time a gang of deer or crows show up and eat half your garden. If any of the neighbors garden (decorative or food) ask them what they've run into.

You might actually be in luck. If it has been growing 'weeds' for decades of neglect, the ground is quite likely rejuvenated and fertile, especially if it was fertile ground to start with.

Will you have access to machines, a plow or rototiller? I am not a huge fan of these, but they can be very helpful in starting new ground. If so, put a thick layer of mulch, manure, organic matter of any sort on top and till it in. Then add another thick layer of mulch on top the ground and leave it on top over the winter. If you don't have machines, no real problem. Just put the mulch on top and leave it there. You can deep dig it or not later if you decide to. Deep digging can be done little by little over years if necessary. It's hard work with no machines!

In spring either till in the top layer or leave it alone on top. I would leave it alone, scrape through it to dirt and plant seeds, leaving the mulch on top, adding more as it gets thin. This helps keep weeds down as well as everything else mulch does.