The first 98 was introduced in 1941 and had a straight 8 engine. The body types were sedan and coupe.
In 1949, the Rocket V8 engine was introduced to the 98 line of cars. A convertible was added to the Holiday Coupe and Town Sedan in the 98 lineup.
These cars got bigger throughout the 1950's. The third generation, sold from 1956 to 1964, had a 394 Rocket V8 in the later years.
The body was changed for the fourth generation, with a less 50's bubble look and more of a 60's muscle/luxury car appearance. The top engine on the 98 was now the 425 cubic inch Super Rocket V8.
By the fifth generation, the 98 looked a lot like the the Delta 88, but you can tell them apart because the 98 had the tailfins that are similar to those iconic symbols of Cadillac.
The 98 was downsized for the sixth generation, sold from 1977 to 1984. The V6 engine was now the standard. Some larger V8's and a diesel V8 were also available as options on some models.
Somehow this car made it to a seveneth and eighth generation, getting lighter and lighter each time.
These cars can be found fairly readily on the used car market. The more recent models sell for just a few thousand dollars, while older classics are collectible and fetch some decent prices. Still, I think these cars are underappreciated by the classic car market - a situation that could change at any time. The best place to find the later models and the classics is on eBay, where parts are also listed in abundance.