Technically called Liber AL vel Legis or Liber Legis, this short book is the fundamental Holy Book of Thelema, the religious/philosophical/magical system for which Aleister Crowley is the Prophet. Whether you believe that he took it down from the direct voice dictation of a superhuman intelligence named Aiwass in 1904 (and interestingly enough many serious Thelemites don't) is immaterial. All Crowley's later work is dependent on the three chapters in this book and therefore you need at least some acquaintance with it if you want to understand his ideas.
There are any number of stand-alone editions of Liber AL by itself. Most of them are fairly small if not actually pocket-size, making them convenient to carry about with you. Actually, there is no urgent reason to pick it up separately since it is also included in several of Crowley's other books, such as the complete Book Four and several numbers of The Equinox. You can also find it in some of the recent 'primers' on Thelema by other authors, such as Lon duQuette's Magick of Thelema. Ideally, according to instructions in the text itself, any proper edition of Liber AL should include a facsimile of the original manuscript in Crowley's not-terribly-legible handwriting. Beyond any possible esoteric meanings embedded in Crowley's scribbles, you can think of this as a way of preventing later editors from altering the text to suit their own agendas.
Any work as perplexing as the Book of the Law simply cries out for some sort of learned commentary and Crowley attempted several over the course of his life. The only one he regarded as completely successful is a one-page 'inspired' Comment, now included in most editions of Liber AL, whose primary injunction is that people should interpret the book for themselves.
This still leaves Crowley's more extensive commentaries, with which he was never satisfied to the end of his life. Some thirty years after he died no less than three separate editions of these long commentaries appeared in print from three different editors each making different cuts in Crowley's text so that all were different and none were complete. John Symonds and Kenneth Grant teamed up to produce Magical and Philosophical Commentaries on The Book of the Law (1974) while Marcelo Motta published The Commentaries of AL (1976) and Israel Regardie called his version The Law is for All (1975). Motta and Grant were motivated largely by a need to legitimate their respective 'pseudo-O.T.O.s' which they were promoting at the time and added their own commentaries to Crowley's. In Motta's case at least this probably did more harm than good, since his commentary displayed his notable paranoia, intolerance, ignorance and vicious diatribes against his supposed 'enemies' for all to see. The Symonds and Grant version is the most complete of the three, although Regardie's was the only one to remain in print for any length of time.
Lost in all this was the fact that, far from being hostile to any abridgement of this work, Crowley had actually commissioned one himself. While not a Magician in any formal sense, Louis Umfraville Wilkinson (1881 - 1966) was a talented writer and close friend for many years. His lack of formal occult training made him the ideal person to edit Crowley's own commentaries down to something both manageable and accessible to the average person. Crowley's instructions were essentially to 'cut out anything that doesn't make sense to you on the first reading,' and Wilkinson set to work. The result, which was never quite finished after Crowley's death in 1947, languished in typescript until 1996 when Hymenaeus Beta completed the project and issued it as... The Law is for All.
Now there is a certain inherent confusion in having two very different versions of the same book out by the same author with the same title... and from the same publisher to boot! It was felt that the title was appropriate to a commentary designed for the newcomer, so essentially the Regardie edition was allowed to go out of print and the Wilkinson edition just replaced it. This does leave the potential buyer with the problem of determining which version is being offered for sale at any given time, especially if you're buying through a catalogue or on-line. Naturally the full bibliographic data will settle the matter seeing as the editors are different and, if all else fails, you just need to ask on which side of 1996 the particular copy is copyrighted. Thankfully, those so old-fashioned as to do their shopping in a store have an even simpler option: the Regardie edition has a white cover while the Wilkinson version is clad in dark purple. It is thus easy to distinguish between them even across a crowded room.
Incidentally, there are plans to publish Crowley's complete commentaries on The Book of the Law. However, considering the agonising process that is Thelemic publishing, there's no telling how long it will take before that happy tome will reach store shelves. So, on the whole, I recommend you get the Wilkinson version. Aside from being the abridgement Crowley actually wanted to publish, it features a full photographic reproduction of the manuscript of Liber AL, an up-to-date bibliography of Crowley's works, and a good index. Naturally you also get the full printed text of Liber AL itself. It is also, by design, the best suited to the beginner.