I recommend the trade journals. Schiff’s Insurance Observer no longer publishes, but the archive is available online at no charge. Dowling’s IBNR, which Alice Schroeder used to write, is good too. I’ve also enjoyed Fortune Magazine’s insurance-related reporting—particularly from Loomis and others in the 70s and 80s.
Other suggestions:
I learn by simply reading annual reports. Try reading the annual reports (and statutory filings) of successful insurance businesses. You can’t go wrong with Berkshire, but I’d also suggest companies like Progressive, WR Berkley, Markel, RLI, etc.
It’s also interesting to learn about the failures. Why do great high-frequency-low-severity insurers like GEICO or 21 Century get into trouble (Loomis wrote a great article on GEICO’s issues in the 1970s). Why did Fairfax have problems? Why did Lloyd’s nearly fail (there are several good books on this)?