Most of this has already been covered here, but I recently stumbled upon an encapsulation I wrote several years ago so I am posting it for one-stop shoppers.
Bill Finger was not well documented in his lifetime by either interviews or photographs (and he received no mainstream obituary). Few instances of his actual words have been published.
The best source: he is quoted rather extensively in Jim Steranko's book History of Comics, Volume 1 (1970). In addition, he spoke on a panel of comics creators at the first "official" comic convention in 1965, which is transcribed in Alter Ego #20, and he is briefly quoted in a short New Yorker piece about that convention.
He is also paraphrased in a now-legendary piece "If the Truth Be Known or 'A Finger in Every Plot!'" This was written by a comics historian named Jerry Bails for a 1965 fanzine and was the first time most fans learned that someone other than Kane was involved in the creation of Batman.
Only two other Finger interviews were known to exist—but neither had been published. One was buried amid the clutter of famous fan Tom Fagan's house in Vermont, the other lost in the bowels of a California university.
In February 2009, the piece produced from the Vermont interview (conducted in the mid-1960s) also surfaced, along with two personal letters written by Finger—the only ones known to exist.
Aside from the published Bails and Steranko pieces, this material was unknown to most of the comics community.