MG Tech Index (mgaguru.com)

MG produced the MGA roadster from 1955 to 1962. This site has been operating by one person for 20+ years and has the biggest wealth of information on how to maintain and fix these cars. The tech section alone has over 4,000 pages of information on these cars.

On the vein of single-model enthusiasts, there's the Jaguar XJS book, hosted in several places: https://www.jag-lovers.org/xj-s/book/