For some background on Lang and his files, one can read the first part of Lang's obituary in the AMS Notices at http://www.ams.org/notices/200605/fea-lang.pdf.
The book "Challenges" was published in 1997. I guess Lang went on to gather documentation regarding several cases dealt with in the book after its publication. Consequently, several related files must have contained much more documentation at the time of Lang's demise than at the time of "Challenges" publication. Besides, "Challenges" does not deal with all the cases Lang has been involved with and interested in.
In addition, the AMS obituary mentions that Lang "has some unpublished books of a political nature".
Are the full (i.e. the whole documentation that Lang had gathered and arranged) files regarding the "Challenges" cases (i.e. those dealt in that book) published somewhere? If not, are they available in some other form? What has happened to Lang's related personal papers?
What about other files not reproduced in "Challenges"? What are they, and where is the documentation?
Same question for Lang's "unpublished books of a political nature".
EDIT 1 (27 November 2012). For some bad reasons, it has taken me several months to decide to write to the Yale librarian. I can now provide a partial answer to my question, that is: these papers have not been deposited at Yale's library. I will follow the librarian's suggestion to write to the University of Texas. If someone there (or elsewhere) reads this post and wants to provide help to locate Lang's papers, please do not hesitate to contact me. I have copied below the message I have received from Yale's librarian following my query. I have hidden the name of the sender, although that may be unnecessary.
November 26, 2012
Dear Mr. Chiche:
I am writing in reply to your e-mail inquiry of October 22, 2012, regarding former Yale professor Serge Lang.
Our on-line catalog shows that the Yale University Library holds a number of Professor Lang’s published works but, if I am interpreting your query correctly, you wish to know about his unpublished writings. The Manuscripts and Archives department does not hold a collection of Professor Lang’s papers. They were offered to us following his death but we declined since the subject matter was outside our collecting areas. We suggested the University of Texas, where a number of mathematical collections are held. I searched OCLC’s World Cat to see if I could locate the papers but I did not find any results. You may wish to contact the library directly to see if the Serge Lang Papers were ever received. Information and assistance is available at http://www.cah.utexas.edu/collections/math.php
If you have any additional questions, please do not hesitate to contact us.
Manuscripts and Archives