Altman Weil recently released a special report "10 Years of Law Firm Mergers". Click here for a copy of the report.

Tons of interesting insight provided by the report, such as:

78% of the acquisitions were of small firms of 2-20 lawyers. Smaller acquisitions can be done quickly and are a low risk way to buy market share.
49% of the acquisitions in the U.S. were in the South (Florida and Texas mostly) and the Middle Atlantic (New York and Pennsylvania led the way). Geography is an important dynamic.
Since 2013, there have been more than 80 acquisitions per year. 85 acquisitions closed in 2016.