There's only one instrument that can be definitively called the oldest in history, and that's the voice. After all, there have been vocals as long as there have been humans! Of course, the vocal music we know today has all been written within the span of recorded history; before then, no one was writing it down. It's a good thing we started, though, because this selection of classical vocal sheet music is filled with some of the most beautiful compositions ever put on paper. A wide range of song is represented here, from music as iconic as Ave Maria to all kinds of lesser-known pieces that will really help you expand your vocal repertoire.
There are two angles you can take to approach the songbooks in this section, and the first is to look based on your vocal range. Some of the compilations are specific to an individual voice type, such as the Hal Leonard Cantolopera Arias for Tenor and Cantolopera Arias for Baritone, or the G. Schirmer First Book Of Soprano Solos. Others, like the G. Schirmer 24 Italian Songs & Arias Medium Low Book/CD, are written in a key that fits into multiple vocal ranges, making them accessible to a wider variety of singers - these are great if you're looking for music you can perform as a group with other vocalists.
Instead of choosing by voice, the other angle you might go with is to pick out a style of music first and then search for books within it that fit your range. In some cases, the same songbook is available in more than one key, so you can use it no matter where you sit on the scale. For instance, the Alfred 26 Italian Songs and Arias Book comes in Medium Low and Medium High versions, as does the G. Schirmer 24 Italian Songs And Arias Voice Book. Other musical styles you'll find in this section include show tunes, carols, spiritual songs, folk and, of course, opera.
So take a look through the whole selection of classical vocal sheet music - you don't need to be a formally-trained opera singer to find something here that you'll enjoy singing. Even if you're at an early stage in your vocal career, there are songbooks to fit your skill level. It's just a matter of finding the right ones, and whether you choose based on your voice or based on your musical preference, you've got plenty of options ahead of you.