Looking for a novel I read in the early 90s; it came from a local library so it may be quite a bit older. Might've been a YA novel, as it was in the same shelf as a bunch of William Sleator novels. All my memory detailed below.
- A spacefaring civilization sends a mission to re-establish contact and possible trade with Earth; they had been in the first stages when WW3 or some other apocalypse began.
- The leader of the mission is a younger woman, humanoid, remember something green about her but can't remember what exactly (hair, skin, or eyes, not sure). Her core team she picks out is small, maybe 3 or 4 people like a tech specialist, a doctor, and a psych/socio specialist. They are assigned a full cruiser ship as support.
- They find Earth has largely recovered but is covered by a lot of wilderness. Humanity is around but what's left is at some tribal, some pre-industrial levels, with very few big cities.
- They establish contact with one of the bigger centers, basically by dropping their shuttle in town square and opening the door, waiting for someone to walk in. The town had apparently studied or worked out that aliens had landed in the past, and worked out a rough first contact protocol if someone landed. I think the logic was to show non-aggression but also determine if the landers had hostile intent as soon as possible. So their rep played canary and walked right into their ship, nude with arms open so he could hide no weapons. He was then blushing when the Green woman unexpectedly returned the gesture in kind.
- A bit of time passes as contact develops. Green woman isn't getting much support from the cruiser, so she requests a bunch of ground bases to be built to keep their crew busy and make them useful. The mission starts making contact with more villages, finding most of them have an odd legend about a savior of humanity, and without fail all of them have a radio of some sort that this savior is supposed to call them on. I think the savior is supposed to be called Lee Lukari, but I may have the name misspelled.
- The team collects a great many of the radios (but not nearly all). They are quite surprised when they all turn on and start requesting a call back so Lee can save humanity. The signal turns out to be from an alien ship on rapid approach. Their own cruiser bugs out because they got caught flatfooted and don't recognize them, claiming a first contact / unknown hostiles regulation that mandates they retreat and analyze. Core team is left on their own with a bunch of half-built bases. The bases still have shuttles, supplies, bits of tech. The biggest thing they have is nigh-invincible energy shields, around each of the bases, but they also have a few personal units too.
- The aliens had arranged the radios for easy collecting of humans, so they can enslave them for use in some sort of farm. The animal they were farming was really odd, and Earth is ideal for it apparently. Strongly implied they pulled strings to cause the fall of man in the first place.
- The animal they farm is constantly producing this fibrous grey trunk of a cable. When the good guys get a bit of it they figure out how to separate the fibers; trying things they weave some into cloth. Something about the fabric produces a pleasure effect on skin, and it's actually directly compared to a drug. They presume the aliens are selling it elsewhere as an exotic narcotic.
- Aliens set up their farms, guerrilla war and much sneakiness ensues. One of the first bits of sabotage found is feeding the animals a particular Earth plant (dandelions, I think) to upset their production - they get diarrhea instead of wonder fabric.
- My brain wants to say the aliens started to be called Unks (short for Unknowns?) but the second thing my brain says is this is ridiculous so I don't know how helpful that is.
- I know there's a bit of relationship juggling in the good guys, but I really don't remember who
- The alien mothership is brought down by the Green woman sneaking onboard, using her personal shield to protect herself, and shouting that Lee Lukari (again, sp?) is having her vengeance. She finds a control center and sets the orbital path to start kissing mountains. As the ship comes down, she can't find the hatch back out. She only barely survives the crash due to the shield tech.
- After some time healing, the "reinforcements" finally arrive, a full combat fleet. Since the ship is downed and the only aliens left are now hiding in caves, they don't have much to do but try to assume command. Annoyed at their antics, she sets herself up as leader of Earth, knowing they can't do much to object since they have a potential war to fight. I think she also offers the wonder fabric they can now produce as a trade item to get the technology to rebuild Earth civilization on a large scale. Manipulating the command into doing what is essentially a space parade (looks like a bunch of moving stars from the ground), she's able to pass off the fleet as her own and convince the remaining aliens to surrender as well.
- I want to say that the cover was colored yellow and green, but I don't remember what it was depicting and don't remember how reliable my memory is here
I have repeatedly tried to investigate the works of Melissa Scott (as suggested by @mkennedy) and found no matches in the plot summaries I can find online. It is possible I'm missing one or more of her works, but there doesn't seem to be a large amount of her books that fit the timeframe.