Before you can access any e-books on your mobile device, you must install a reader app. Keep in mind that there are dozens of options to choose from and this guide only offers one among many. Each one has its own relative merits so it's up to the individual user to find one that suits his or her best.
Overdrive is a good choice as it's not only a general e-reader, but it's also the standard platform for delivering e-books and audiobooks from local libraries. With Overdrive, you can read any e-book you download and also use it to borrow library e-books and audiobooks. It's available on these platforms: iPhone, iPad, Windows Phone, Android, Kindle, Nook, Play Book, Windows, and Mac.
The staff at the Massachusetts Trial Court Libraries has done attorneys and law students a terrific service by making a number of court rules available as e-books. These will come in handy if you find yourself in a courtroom and don't want to lug around a print copy of court rules to consult. Depending on which reader you use, these e-books are fully searchable and include tables of contents for browsing. The following e-books are available for download on their website:
Mass. Rules of Civil Procedure (11/30/12), Includes amendments effective 1/1/13
Mass. Rules of Criminal Procedure (09/10/12)
Mass. Rules of Appellate Procedure (04/02/12)
Mass. Rules of Domestic Relations Procedure (03/07/12)
Mass. Juvenile Court Rules and Standing Orders (04/23/12)
Mass. Land Court Rules: Includes Rules, Standing Orders, the Manual of Instructions and the Guidelines on Registered Land. (04/18/12)
Mass. Superior Court Rules(11/30/12)
Mass. Guide to Evidence (03/19/12)
Boston Public Library and Cambridge Public Library both have e-book and audiobook collections that you can download straight from your mobile device with Overdrive. A quick Google search for your local library should also turn up a relevant site.
Additionally, Project Guttenberg probably has the internet's largest collection of e-books, sourcing its collection from works now in the public domain. If you want to read Melville, Tolstoy, Austen, and many other long-dead writers on your mobile reading device, this is your best bet. Visit Project Guttenberg and avoid scams.
Both of these options are ideal if you want to avoid purchasing e-books while also respecting copyright law.