Kluwer, with head office in Hague and offices in other countries including UK and USA, are known to be a very famous and respected name for law books. They also publish International Encyclopaedia of Law in different branches of law, which are being edited by R. Blanpain, a distinguished former Professor of Law from University of Leuven, Belgium. Allen & Unwin are also quite famous,

You might want to look at:

http://lawlib.wlu.edu/LJ/index.aspx?mainid=118

http://guides.library.harvard.edu/content.php?pid=68282&sid=512184

also look at:

Ranking Law Journals and the Limits of Journal Citation Reports by Theodore Eisenberg & Martin T. Wells

Hope this helps

@Debi, thank you for your answer. Kluwer is certainly good or fabulous in text and "trade" publishing, but in respect of scholarly monographs, on all the exchanges I had with colleagues it was lesser regarded than most (important) universities publishing houses, and sometimes lesser than comparable global publishing houses. An article ascertaining how academics rank law publishers in Spain, returned more or less similar classifications, and a sort of ranking for tenure purposes published by a Dutch University indicates that the publishing a monograph with Kluwer gives an applicant 3 points, while the publishing with a prestigious University Houses gives the same applicant 4 points. I guess that this was so because Kluwer focuses so much on what we call "trade" books, and probably this European subjective "classification" is not the same elsewhere. But probably, I should rather ask about the classification of academic law books publishers, not generally about law book publishers.

@Arturo, thank you for the Eisenberg & Wells' pointer (I use WLU mostly to have an idea about what journals exists, not necessary as a reliable indicator); I got some interesting materials just"google-ing" it.