I'm trying to remember a series that I read a couple of books from when I was a child in the 1980s. I would say they were more children's novels than "young adult." I remember I read the first book in this series, and then the third or fourth, since the library did not have the immediate sequel.
There were six main characters, and four of them were ghosts. Most of the ghosts were recently dead teenagers, although one was an adult (a general contractor who was murdered by being pushed off the balcony of his high-rise office). The two living main characters were the two best friends of two of the dead teens. The living and dead nerd best friends had managed to measure the tiny disturbances that the ghosts could cause, and use them to communicate through a super sensitive computer keyboard.
The ghosts can travel around unseen, but little else. They can't pass through objects or fly, and can only communicate with the living through their computer setup and a slightly tingly touch. However, this is enough for them to help their living contacts solve crimes. I think the first crime was a bank robbery (and one of the robbers was killed by police at the end of the book, coming back as a evil ghost in the later volume), but I don't recall the plot of the other book I read.