Every bibliographic management program has a way to associate a unique identifier with every book or publication that it keeps track of. You will need to read the program manual in order to find out what this code looks like in the software that you use.
Some examples of bibliographic codes are as follows:
Citation: The bibliographic code is contained in the field "Cite Key" of the bibliographic record; e.g. "Smith 2005".
RefWorks: The bibliographic code is the same as the record number that your new publication is assigned upon creating a record.
EndNote: The bibliographic code is the same as the record number.
BibTeX: The bibliographic code is generally a combination of last name and year of publication, together with a random set of letters to distinguish publications from the same year, e.g. "Arnold:1966uq"
If you wish to use TextCite together with a bibliographic manager program, like those listed above, you will need to take care to make sure that TextCite knows the unique bibliographic code associated with a book, and that the publication record in TextCite is correctly updated in order to store this code.