Please help me with understanding the following sentence. The firms being investigated read like a who's who of Wall Street and the City of London
It means that when the names of companies are looked at, it is almost as if you are reading the "Who's Who" of important financial companies. It's telling you that it's not just little companies being investigated, it's the major financial institutions.
In case it's not clear from the written version, "read" here is the past tense of "to read", the present tense. However, the writer could also have said "....being investigated READS like a....."