With a used car, what matters is not only whether a service book exists, but what it actually shows. A good history is consistent and logical, not just a random pile of papers.
Check whether services were done at sensible intervals. If oil change gaps are very long or several years are missing, be cautious. Also look at whether the mileage develops logically over time.
It is useful to see larger jobs as well, such as brakes, belts, suspension work or a battery replacement. These give you a better idea of how the car has been maintained and what costs might be coming soon.
If the seller can present the service history clearly and answer questions confidently, that is usually a good sign. If the information is messy or incomplete, it is worth being more critical.