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: LA County Library Career Opportunitites 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. Although previously the results of these surveys were made publicly available every year and provided a snapshot of the state of affairs for public librarians, the data are now behind a paywall, i.e., only available via a premium subscription.
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 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 useful information about library jobs.