Here is a nice short overview where Mandarin tone sandhis: http://web.mit.edu/jinzhang/www/pinyin/tones/index.html
If you want to read into the details I have found the following a very good source (from the father of another romanization): "Yuen Ren Chao: A Grammar of Spoken Chinese. University of California Press, Berkeley, 1968, ISBN 0-520-00219-9."