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://lacountylibrary.org/ 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.
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 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.