There is no set range of money you'll get working at a library during the summer. As libraries are usually non-profit, they can't afford to have large staffs or even hire summer help. Therefore, the lower range of pay for working at a library during the summer is zero. However, having a volunteer position can really help to build your resume and give you the experience you need to get a job later in the year or be accepted into college. So, don't discount jobs simply because they are not paying opportunities.
That said, there are paying opportunities as well. You pay rate is largely determined by the library's budget. The more traffic the library gets, the bigger their budget will be, in general. Starting rates are usually just above minimum wage, but if you return every year, this rate will only rise. If you have experience or are working on a degree in a related field, like English, you can expect a slightly higher pay rate, especially if you negotiate for it.
As always, it is important to look at the benefits outside of pay rate. Although it may be slightly lower than some of the other summer jobs, you get to work inside instead of outside in the heat, the work is not hard manually labor, and you have quick access to whatever books you'd like, including ones that the library just got in. You also may have your choice of free books that do not sell when they library has a book sale.