On 13 March 1996, Don Knuth met with members of the Dutch TeX group (NTG) for a Q&A session.
During this session several questions came up that alluded to troff. The transcript of the entire Q&A session appears in TUGboat 17:4, pp.342-355, with the immediately relevant material starting on p.348. But the underlying philosophy is laid out in the answer to the question from Frans Goddijn that starts on p.347.
After some discussion that mentioned troff, Andries Lenstra asked "Why didn't you start from troff? It was completely inappropriate?" This perhaps doesn't completely answer your question, but the short answer is that troff at that point was layer on layer of patches, and Knuth felt that another patch wasn't the answer -- time to start all over. (After all, as he pointed out a little later, he scrapped TeX78 when he decided that he had found a better approach, and wanted a clean implementation.) It's not so hard to understand when you realize that he's a perfectionist.
Quite a few of Knuth's Q&A sessions were transcribed in TUGboat; to see all of them, look for the heading Knuth interviews and Q&A sessions in the list of contents by keyword.
I think the comments above make as good an answer as any would so I've taken the liberty of assembling them into a single CW answer.
The TeX lion was designed for the illustrations of the first TeXBook. Duane Bibby is the artist. He describes the genesis of the idea in a 2006 interview this way:
Q: How did the lion motif come about?
A: During that first meeting, Don [Knuth] showed me some writing he had
done that had been published in Mad magazine. That not only greatly
impressed me, because I had a fair stack of rejection slips from them,
but helped in thinking about perhaps using a character to bind things
together — somewhat similar to a job I'd just completed with Eleanor
Mennick, the design director at Fearon. Don liked the idea, but we
didn't have a clear vision at that time of exactly what sort of
character. The obvious was, we thought, a kind of computer guy, of
which I did some exploratory sketching around. But, as my wife
Jeanette Ahlgren noted as I doodled in our cabin in the trees, also
kind of boring. I'd also had a chance to read the manuscript and found
the tone light and engaging even though I knew nothing of computer
software. I think that influenced the approach too.
Various animals came to mind and pad, but a classic lion finally began
to pop to life. A possible source of the lion idea was a very large
Maine Coon cat — a rather large breed of house cat — that was
wandering around. It had been abandoned, was looking for a new home,
and was giving us new arrivals the look over, trying to decide if he
would adopt us. He later did. ...
I tried the lion sketches on Don, which he liked right off, and we
then began working out each chapter idea which further defined the
character. Later when it came to Metafont, Don felt the lion needed a
mate and so that made it easy.
The LaTeX Book and The Metafont Book were also illustrated with Bibby's lions, which further enforced the brand.
The TeX Talk logo designed by Pablo is an homage to Bibby. TeX StackExchange users would love to have the lion further incorporated into site graphics, but the money required to hire Bibby has not surfaced. See
CTAN has a lion illustration that is liberally licensed; Jim Hefferon owns it and requests that proper attribution (to the artist) be made. In fact we used it for our captcha screens. (The linked page in turn links to several scanned versions that are available for appropriate uses.)
Some more history: At the "coming out party" for the first hardbound set of Computers & Typesetting, at the Computer Museum in Boston, 21 May 1986, Don Knuth concluded his remarks with a comment on "why a lion?" (p.98). The introductory remarks by Knuth's editor, Peter Gordon, provide more general background about the occasion.
Best Answer
I don't know of any text that mentions Jill's help for the TeX language, the implementation, etc. But if the question is extended to the complete project TeX, METAFONT, and Computer Modern her help is documented.
Besides all the support that she gave Donald Knuth in their relationship---the NYT article mentions that he starts to be active at night, sleeping during the day; what does that mean for the partner?---he mentions a couple of times in his papers her contributions.
First in the article based on his his Gibbs lecture of 1978, in which he talked about his research in Mathematical Typography. The section Acknowledgments starts with the words
(page 59 in Digital Typography (DT)). Earlier in this article he wrote
In this part he discusses the problem to draw the letter `S'; page 48 in DT.
On page 64 of the book Companion to the Papers of Donald Knuth he says
That was an early stage, in which he tries to get the letter forms used in his books The Art of Computer Programming.