After a pencil the first drawing tools I learned to use was a crowquill nib pen and india ink. As a kid I poured over ink drawings in comic magazines like Marvel Comic's Savage Sword of Conan and the Warren Magazines like Creepy and Eerie. After finding out that the artist's used both brush and pen I started doing the same.
Latter I learned about Illustrators like Edin Austin Abbey, Joesph Clement Coll, Franklin Booth, Harrison Cady, etc. I also looked at pen drawings (and etchings) by artist's like Tiepolo, Rubens, Rembrandt, Holbien, Zorn and that list is still growing.
What I like about pen and ink is the challenge to say so much with so little, just black and white mostly. You can, of course, use as many color inks as are available. I choose to use one color ink at a time, either black or sepia. I might add a wash to the drawing.
One of the ways I would practice is by making a bunch of pencil drawings or doodles in my sketchbook at night so in the morning when I wake up I would go over these drawings in ink.
Another challenge is to draw without laying in any pencil drawing, Just begin sketching with ink and pen or brush. It forces me to really concentrate on the drawing because I can't take back a single line.