If you want your Sim to master the guitar skill and learn all of the songs, Don't just play the guitar to reach level 10 because you want be able to learn new songs from books.

To learn the new songs just reach level 5 in the guitar skill, then put a level 5 book in your sims inventory and click on the guitar and select new song (to learn it).

When you reach level 6, do the same thing again for level 6 books. Keep repeating this process until you reach level 10 and your Sim will now know every song you've taught it.