"Panel Data Analysis" entry in the Encyclopedia of Research Methods for the Social Sciences, edited by Michael Lewis-Beck, Alan Bryman and Tim Futing Liao, Sage Publications, 2004. 
ISBN: 9780761923633  

"Panel Data" entry in the Encyclopedia of Statistical Sciences, S. Kotz, C.B. Read and D.L. Banks, editors, John Wiley and Sons, 1998, pp. 514-517.