Maybe a 3.5, but I'll round up because I laughed out loud several times and I'm hard pressed to do more than a smirk when I find something I'm reading funny.
If you've never read a Mary Roach book before, her work is like this: she researches a bunch of scientific studies about a particular subject, and then presents them to you with witty prose and tons of oddball stories along the way. It's quite enjoyable, but beware that there is a fuck-ton of name dropping constantly, to of course credit all the sources and people she found information from, and you have to be one who enjoys reading about sciencey type stuff at least on a cursory level.
This is her book on ghosts. She is a skeptic, hoping that through her research something can be proven to her. I am not a skeptic, I just don't believe anything. I don't sit and wonder, I personally feel confident in my decision that nothing is going to happen to me when I die, my brain will shut down and so will any control, memories, or anything, and that will be that. That's cool though, I don't care one way or the other (though my ex was just terrified by the notion that I thought this. Meh.) This book does nothing to try to convince you of any ghosts or afterlife either, because almost all experiments are either failures or serve the purpose of debunking anything that seems to be a ghost.
I think the issue with this book, and what makes it not as good as Stiff, or even Bonk, is that it is based on something that can't actually be proven one way or the other, intangible and untouchable. With the science presented in Stiff, we actually learn things that are exactly as they are. With these stories, it's like "well that was the experiment, and nothing happened." There are many stories like that in the book. It's interesting to see what lengths people will go to, but in the end, it's all the same, there's no proving it.
What I did like, is the silly stories of people acting like complete fools. There was plenty of that. And in fact, since I don't have much more to say (I enjoyed the book, you might enjoy it too if you like Mary Roach and science, but I recommend much, much more her book Stiff first), I'm going to leave you with my favorite passage from the book, which I typed up just now just so people can have a laugh:
Is it possible to dress up like a ghost and fool people into thinking they've seen the real deal? Happily, there is published research to answer this question, research carried out at no lesser institution than Cambridge University. For six nights in the summer of 1959, members of the Cambridge University Society for Research in Parapsychology took turns dressing up in a white muslin sheet and walking around in a well-traversed field behind the King's College campus. Occasionally they would raise their arms, as ghosts will do. Other members of the team hid in bushes to observe the reactions of passerby. Although some eighty people were judged to have been in a position to see the figure, not one reacted or even gave it a second glance. The researchers found this surprising, especially given that the small herd of cows that grazed the field did, unlike the pedestrians, show considerable interest, such that two or three at a time would follow along behind the 'ghost.' To my acute disappointment, "An Experiment in Apparitional Observation and Findings," published in the September 1959 Journal of the Society for Psychical Research, includes no photographs.
Several months later, the researchers revised their experiment, changing the venue and adding 'low moans' and, on one occasion, phosphorescent paint. One trial was set in the graveyard right off a main road and clearly in the sight line of drivers in both directions. Here observers hid in the bushes not only to record reactions, but to 'avert traffic accidents' and 'reassure anyone who became hysterical.' But again, not a single person of the hundred-plus who saw the figure thought it was a ghost, including two students from India. "Although we are superstitious in our country," the men told one of the researchers, "we could see his legs and feet and knew it was a man dressed up in some white garment."
In their final effort, the research team abandoned traditional ghost-appropriate settings and moved the experiment into a movie theater that was screening an X-rated film. The author of the paper, A.D.Cornell, explained that the X rating was chosen to ensure no children were traumatized by the ghost, as though that somehow explained the choice of a porn theater as a setting for a ghost experiment. This time the 'ghost' walked slowly across the screen during a trailer. The phosphorescence was not used this time, and presumably low moans were deemed redundant. No mention is made of the specific images showing on the screen behind the ghost, but clearly they were a good deal more interesting: The audience was polled after the film, and forty-six percent of them didn't notice the man in the sheet. Among those who did, not one thought he'd seen a ghost. (One man said he'd seen a polar bear.)