As you might imagine, the salaries of librarians vary greatly throughout the country. A public librarian at a large, urban library in a city with a high cost of living may have a higher starting salary (see: http://www.colapublib.org/careers/ for an example) than a public librarian in a small, rural library.
Library Journal conducts a yearly survey to determine current median salaries in libraries nationwide. The results of these surveys are made publicly available every year and provide a snapshot of the state of affairs for public librarians.
In addition to this, most states collect data on employees, including librarians’ salaries. ALA has compiled a useful list of states that provide this sort of information as well as direct links to statistics.
Though there is not normally a tenure process in public libraries, as government employees, librarians do tend to be rewarded on the basis of seniority.
Check out the job postings and salary ranges above and on the previous page to gain insight into the average salaries for various positions in a public library. You can also look at GlassDoor, which has salary, real interview data, and other information.