Born in NYC, she grew up in New Jersey, lived for years in Frankfurt, West Germany and now lives in London. (Possibly Shepherd's Bush, London.)
http://booksforkeeps.co.uk/issue/8/childrens-books/articles/other-articles/picture-book-people
(brief bio - you have to scroll down a bit)
...Volunteer work brought her to Europe straight from school, as an American Red Cross worker in 1961. She spent many years in Frankfurt and, after resigning from the Red Cross, went to art school in Offenbach. There, she began to consider doing children's picture books, and on a trip to London to do the rounds of publishers, Ernest Benn offered her her first book.
The idea for doing a book with teddybears as the main characters grew from her early experiences with publishers. 'At the time I started, I got so many rejections from publishers who said my creatures weren't sweet enough. so I decided to do the bears almost out of revenge. I thought I would invent a really cuddly creature, but although they are cuddly, they're not that cuddly. I think they have a certain licence - they're not really like children, but they're not really like adults either, and that's why I think they're appealing.'...
https://www.google.com/search?q=susanna+gretz+books&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj607Ho8MXWAhVpz1QKHfi7CtAQ_AUICigB&biw=1280&bih=553
(book covers)
https://www.google.com/search?q=%22susanna+gretz%22&source=lnms&tbm=bks&biw=1280&bih=553
(synopses)
https://www.onemorestory.com/people/susanna-gretz
(booklist in alphabetical order)
https://www.google.com/search?biw=1280&bih=553&q=susanna+gretz+kirkus&oq=susanna+gretz+kirkus&gs_l=psy-ab.3..33i160k1l2.7867.8575.0.8761.6.6.0.0.0.0.185.355.0j2.2.0....0...1.1.64.psy-ab..4.1.184....0.sLEryiNxFS0
(seven Kirkus reviews)
https://www.goodreads.com/author/show/297863.Susanna_Gretz
(reader reviews)
https://www.google.com/search?q=%22susanna+gretz%22&source=lnms&tbm=vid&sa=X&ved=0ahUKEwjszP_u8sXWAhWJgFQKHVdVBVoQ_AUIDCgD&biw=1280&bih=553
(videos about her books)
WRITINGS BY THE AUTHOR (only up to 2001 or so):
SELF ILLUSTRATED
Teddy Bears 1 to 10, Follett, 1969, reprinted, Four Winds Press, 1986.
The Bears Who Stayed Indoors, Follett, 1970, 2nd edition (with Alison Sage), A. & C. Black, 1986, published as Teddy Bears Stay Indoors, Four Winds Press, 1987.
The Bears Who Went to the Seaside, Benn, 1972, Follett, 1973.
Teddy Bears ABC, Follett, 1975.
Ten Green Bottles, Penguin, 1976.
(With Sage) Teddybears Cookbook, Doubleday, 1978.
Teddy Bears' Moving Day, Four Winds Press, 1981.
Teddy Bears Go Shopping, Four Winds Press, 1982.
(With Sage) Teddy Bears Cure a Cold, Four Winds Press, 1984 (published in England as Teddy Bears and the Cold Cure, Benn, 1984).
It's Your Turn, Roger!, Dial Books for Young Readers, 1985.
Hide-and-Seek, A. & C. Black, 1985, Four Winds Press, 1986.
I'm Not Sleepy, A. & C. Black, 1985, Four Winds Press, 1986.
Ready for Bed, A. & C. Black, 1985, Four Winds Press, 1986.
Too Dark!, A. & C. Black, 1985, Four Winds Press, 1986.
Roger Takes Charge!, Dial Books for Young Readers, 1987.
(With Sage) Teddy Bears Take the Train, Four Winds Press, 1987.
Roger Loses His Marbles, Dial Books for Young Readers, 1988.
AUTHOR AND ILLUSTRATOR
Duck Takes Off (1st American ed.), Four Winds Press, 1991.
Frog in the Middle, Four Winds Press, 1991.
Frog, Duck, and Rabbit (1st American ed.), Maxwell Macmillan International, 1992.
Rabbit Food, Candlewick Press, 1999.
Lenona.