I have never used it and stopped buying it when HRA, under Rod Pollock, stopped printing the Stud Book. Originally it was replaced with a CD but HRA was still asking about $100 for the useless piece of plastic. I never bought one but I recall using CD-based catalogues for work at the time, much of which were ho-hum. The last book I bought was volume 35 for 2004 and I refused to buy any more.
At the time the cost of producing such a high quality hardback bound book was proving prohibitive for HRA's small run of a few hundred. it listed stallions and mares, their breeding, their breeders and progenies winning times. For stallions it listed winning progeny and best times; sires they sired; winning progeny of dams they sired; black type performers; and live foal numbers by season.
The broodmare section listed every mare that had either left a winner, had a foal registered or named since the previous foaling. Each entry gave the mare's best race time, breeding, year of foaling and breeder. All breeding records - foals winners and stallions they were served by each season and the breeder of each foal is listed.
I would imagine the on-line version would give far more information - it would be able to cross reference so many tables and publication. One would hope that Year 1 is not the 1980/81 season as it is on the HRA website!