The sequel to both series! Best if you read those first, starting with Zita, to understand all that happens. Spoilers ahead for the earlier works.
This is not merely the cameos that were in the Jack series -- it opens with Lilly and the goblins chasing off a giant -- and then what happens with the giant (a threat to Earth) -- but then Jack and Zita in the grocery store buying for a picnic. Meanwhile, Piper and Madrigal face a high-speed chase and mysterious government agents. And then the picnic is interrupted by a gate appearing the garden.
Robot Randy tries to hold it off. Our heroes attempt to get help by having Mandy's dragon get them into the Inbetween. And only the five make it through in time -- Jack, Zita, Lilly, Joseph, and Mandy. So we have a splitting of forces as Joseph and Mandy (with some goblin help) try to keep the giants back, and the other three head off for help.
It involves keys, hieroglyphics, a fairy tale trope played out as it is in some folktales, a bigger giant, sleeping about a fire, a dead world, people certain that Zita the Spacegirl can handle a giant, a vendor who gives them something, and more.