To directly answer your question, the Bible of securities lit is probably the PLI treatise. (
https://legacy.pli.edu/content/Treatise ... ?ID=297611)
But my real answer is that you shouldn't in a million years go right for that. It's too much to take on and will do a poor job of orienting someone new to the field. What you really want is a chicken soup for the junior securities lit associate soul. And that is the much shorter book in the concepts and insights series. (
https://www.amazon.com/Securities-Conce ... b_title_bk) That will orient you pretty well. Importantly it will also help you glimpse the offering and annual disclosure process. You can get by doing securities lit without understanding those, but you will definitely miss arguments and misunderstand facts if you do not.
An immediate free resource is an outline from Wachtell about the major private causes of action. (
http://www.wlrk.com/docs/OutlineofSecur ... es2014.pdf) Again, this does a poor job orienting you but it's free and instant. It is also a little out of date.
Finally, GDC does an end-year report on securities lit. Here is the 2018 report. (
https://www.gibsondunn.com/2018-year-en ... on-update/) Google the past few years and it will give you a sense of what the edge issues are.
Final thing on will say is try to get a sense of what kinds of suits the firm is involved in. If it's a small defense firm, then you're almost certainly focused on insider trading or other technical violations. If it's a plaintiff's firm, you probably are broader but centered on class actions under Section 10(b), 11, 12, and 14. Only large firms defend those class action claims.
Posted anon because this clearly outs me as a securities litigator which could be pieced with other info about me.