I'll send you a more detailed answer per e-mail, but perhaps your request touches a subject which is also interesting for other people; so I put part of my answer in here.
As far as I know there is only one book of Max Wertheimer still available in bookstores in English: PRODUCTIVE THINKING (Enlarged Edition, Edited by Michael Wertheimer, The University of Chicago Press, Chicago 1982, ISBN 0-226-89376-6, paperback). As this is one of the most important and inspiring books of Max Wertheimer I would strongly recommend it.
In 1995 David J. MURRAY, Professor of Psychology at Queen's University, Kingston, Canada, published a very interesting book: GESTALT PSYCHOLOGY AND THE COGNITIVE REVOLUTION (Harvester Wheatsheaf, New York - London - Toronto 1995, ISBN 0-7450-1186-1, paperback). He points out the topicality of Gestalt theorists' ideas for modern cognitive science and argues that science needs to re-evaluate many of Gestalt psychology's ignored insights. Important parts of his book are dedicated to a critical discussion of Max Wertheimer's work.
Indispensable for a thorough and vivid understanding of Max Wertheimer's work after his forced emigration to the USA are Edith and Abraham LUCHINS' collections of Wertheimer's seminars at the New School: WERTHEIMER'S SEMINARS REVISITED. There are several volumes, dedicated to specific areas (all of them were published in the series: Psychology Department Reports, State University of New York, Albany; publishing years are given below): 1) Wertheimer's Seminars Revisited: Problem Solving and Thinking. Vol. I-III (1970) 2) Wertheimer's Seminars Revisited: Problems in perception. Vol. I-V (1972-1974) 3) Wertheimer's Seminars Revisited: Expression of personality and memory for impressions (1978) 4) Wertheimer's Seminars Revisited: Impression and conceptions of personality (1979) 5) Wertheimer's Seminars Revisited: Problems of psychology of character and personality (1979) 6) Wertheimer's Seminars Revisited: Character, personality, and the social order. Vol. I-III (1979-1980) These volumes should be available via libraries.
Last but not least I would recommend a look at the 'Links to Gestalt Related Sources' page of the web-site of the international Society for Gestalt Theory and its Applications (GTA) http://rdz.stjohns.edu/~gerhard/gta/links.html where you will find a lot of links to Wertheimer-related sites in the web. The main page of this web-site is http://rdz.stjohns.edu/~gerhard/gta/gtax.html Good luck for your work! Perhaps it will qualify for the WOLFGANG METZGER AWARD 1999? Should you be interested in that have a look at http://rdz.stjohns.edu/~gerhard/gta/award.html