There are elements that define a good voice. Many interesting singers don't have what would be clinically classified by voice teachers as having great voices but they have something. They may or not have 1. Good diction 2. Good vowel sounds 3. Ample breath support
4. Flexible and consistent register 5. Ability to navigate the "passagio" (the place where the chest voice becomes a head voice)
6. Have good dynamics, 7. Intonation 8. Ease in singing, 9. Ability to maintain a "legato" sound, 10. Last but not least, good interpretive skills for the music they sing. 11. Also, a healthy and natural vibrato.

Try to get voice lessons with the best teacher you can particularly if you are a professional singer. It'll keep your voice from dying.

Two good books widely different but instructive are: The Art of Singsing by Richard Miller. The other (controversial) Singing For The Stars by Seth Riggs. They both have some important information.
But there is no substitution of getting good vocal lessons.

Each singer that you like has one of the above characteristics at least. For example, Woody and Pete..... good diction. Burl had a lot of voice training which served him well. Black singers interpret their music often phenomenally. (Mahalia, Louis, Lady Day, Stevie Wonder et.al). Opera performance seems almost diametrically opposed to the intimacy of folk music. Oranges and apples. (Both great when great). Jazz singers have remarkable flexibility in their registers and vocal range. (Ella, Sarah Vaughan). Judy Collins can sing.
John Jacob Niles had one of the weirdest voices ever known and yet when you saw him live and get into his peculiar style, he is arresting (an acquired taste, believe me). Russell Oberlin floats. Dyer-Bennett coordinates thoughtful singing with his guitar accompaniments. Josh White integrates playing and singing so that you sometimes don't know where the voice leaves off and the guitar begins. Lead Belly and the blues shouters can really belt with support......well you can go on and on but every singer that is somewhat successful and received by an audience has something that carries them.

Frank Hamilton