Klein landed his first freelance lettering job in the fall of 1977, and by late 1977 was entrusted with an entire issue: Firestorm #1.

Books he worked on during this period included Alan Moore's Swamp Thing, Batman: Year One, and Detective Comics; as well as his own scripts for Omega Men.

In the 2000s, Klein lettered all the Alan Moore America's Best Comics titles, including Promethea, Tom Strong, and Top 10.

Klein is most known for his work on Neil Gaiman's Sandman, where he developed very distinctive dialogue balloons and lettering for various characters, especially Dream and his siblings.

Destiny's speech was simply italic (really just slanted) [...] Neil had a specific idea about Delirium's style, that it represent a sort of mad variety, getting louder and softer, like something going in and out of focus.

In this guide, Klein gives a thorough review of how he mentally approaches a page and then goes about doing the actual lettering, either by hand (pen and ink), or by use of the computer (Adobe Illustrator).

[6] From 1995 to the present, most of Klein's logos have been done on the computer (although many began as hand-drawn sketches that were scanned and traced in Adobe Illustrator).

[7] Notable logos from this period include Challengers of the Unknown, Silver Surfer, Legionnaires, Iron Man, the Legion of Super-Heroes,[7] The Life and Times of Scrooge McDuck, Albion, Witchblade, Terra Obscura, and Tom Strong.