There were 3 types of legal books: Cases (info on cases tried in courts), Codes (actions by legislative bodies), and Analytics (books written about how to interpret the cases and codes).

Legal Publishers would publish books that covered specific subsets of cases, codes or analytics and sell them to law firms.  These were an absolutely necessary tool and each attorney needed to have access to the books which covered the law that they practiced.  As the info in these books changed with new cases tried, codes legalized and analysis upon all of these, there had to be a way to get the new info into the hands of attorneys.  The books themselves contained pockets in the front and back cover.  As new info became available, the publishers would produce "pocket parts", which were printed and sent to customers (with an invoice for the new info).  When the amount of "pocket parts" became too unwieldy to fit comfortably in the book, then the publisher would print a new addition of the book and ship that to customers (once again with a new invoice).

This made for an incredible business model for the companies that did this---basically West Law and Thompson Legal Publishing.  They could basically forecast their annual business down to the last penny years in advance as they could plan when/how many pocket parts and new books they would publish in a given year.  And the attorneys had to have them as they had to have the latest and greatest info available to do their work.

Then technology made a radical shift in this business model. First with CD-ROMS, then with workstations in the law offices, and finally, with the internet, this information was made availabe in digital format and attorneys no longer had to wait for pocket parts or new books to be shipped to them.  It was made available almost as soon as the actions by courts, legislative bodies or analysts happened.  Law offices now purchase subscriptions to these services (largely run by Thompson-Reuters and Lexis-Nexus).

The books still exist and are published in some small quantities, but they are now referred to as "wallpaper" for the law offices as they give an old-school look, but are not used for much, if any, real legal work now.

Sadly, I know far too much about this business as I had one of the legal publishers as a client for many years about 25 years ago.