It seems likely that this was the mortuary book of James Crowe, who served as the coroner and owned a mortuary. This would explain it being housed with the country records.
If you browse through this volume you will find it includes deaths that occurred elsewhere. Some
died elsewhere and were buried in the county. Some died in the county and were buried elsewhere. This is in keeping with it being the dealings of the mortuary rather than the coroner or the county.
Whatever the original intent, what you have here is a recording of some of the earliest deaths (1880s-1920s) in San Mateo County. This is a full transcription of what is in the book including name, age, birth place, death date, death place, marital status, sex, race, occupation, cause of death, physician, cemetery, date of burial and mortuary.
If you do not find the record of someone who supposedly died in here, be sure to check both the San Mateo County Death Register (an index to the earliest death records will be available later this year on SMCGS) and the San Mateo County Coroner's Index. Other possibilities for early death records are the newspapers or
church records. "
Check out the other SMCGS Databases and good luck with your research.