Wordless music is wonderful for studying - your brain is less focused on the words in the music and more focused on the words in your textbook, but has some kind of "background noise" to drown out other distractions. Wordless music is also wonderful for reading or napping. Songs include "mainstream classical music" (e.g. Frank Ticheli, Percy Grainger) and obscure game soundtracks (e.g. Ultima IX: Ascension).