It is not necessary to be wealthy to collect Dr. Seuss. Most titles can be found in very good or better condition at a reasonable price.
However, if you want to collect first editions, especially of his early books, you will need deep pockets. Even if the price is no object, some first editions are simply hard to obtain. If you decide to collect first editions, the definitive guide is First Editions of Dr. Seuss Books written by Helen Younger, Marc Younger, and Dan Hirsch. Although only published in 2002, it is out-of-print, and used copies sell for over $100.
Linda and Stan Zielinski's Children's Picturebook Price Guide, 2006-2007: Finding, Assessing, & Collecting Contemporary Illustrated Books has a section on Dr. Seuss first editions and is much more affordable. The Zielinski's web pages, 1stedition.net/drseuss.html and 1stedition.net/blog/category/first-edition-identification-points/seuss-first-editions/ contain valuable information. While they highly recommend the book by the Youngers and Hirsch, they note that their research sometimes leads to different conclusions.
Judith & Neil Morgan's biography Dr. Seuss & Mr. Geisel is not only useful but a fun read. Theodor Geisel: A Portrait of the Man who Became Dr. Seuss (Lives and Legacies Series) by Donald E. Pease is shorter and relies on the Morgans' book for some of its biographical information but is also useful.
Collecting Dr. Seuss books fall into several categories:
Books that were written and illustrated by Dr. Seuss during his lifetime:
These 44 books form the core of any Dr. Seuss collection. They begin with To Think I Saw It On Mulberry Street (1937) and end with Oh, The Places You'll Go (1990).
Books illustrated by Dr. Seuss but written by someone else:
In 1931, Ted Geisel illustrated a collection of children's sayings compiled by Alexander Abingdon which was published with the title Boners by Viking Press. More Boners, also illustrated by Seuss, was published later that year.
Books that were written by Dr. Seuss but illustrated by someone else:
You can easily identify these books because Seuss uses the pen name Theo LeSeig. LeSeig is, of course, Seuss's real last name, Geisel, spelled backwards.
Beginner Books series:
Beginner Books was a subsidiary of Random House. Ted and Helen Geisel were part owners of the company, and for a time, Geisel was the editor. Several Dr. Seuss and Theo LeSeig books were part of this series. The first Beginner Book published was The Cat In The Hat by Dr. Seuss (1957). These books were published in a much smaller format than earlier Dr. Seuss books.
Other well-known writers and illustrators whose books were included in this series were Jan and Stan Berenstain, P. D. Eastman, Walter Farley, Al Perkins, and Mike McLintock. Bennet Cerf, a longtime editor of Random House and instrumental in starting another collectible series of books, The Modern Library, contributed some books to this series also. The series was a response to Rudolf Flesch's book, Why Johnny Can't Read (1955) and was meant to provide a substitute for the widely used but bland Dick and Jane readers.
Grolier Children's Books reprints of Beginner Books
These inexpensive versions of the above books reached a different audience, were very successful and made enormous profits for everyone concerned.
Bright and Early Books
Bright and Early Books were 32 pages, half the length of the Beginners Books, used a smaller vocabulary and much more repetition. Seuss's The Foot Book (1968) was the first of these pre-readers. Other Seuss titles include Mr. Brown Can Moo, Can You? and Marvin K. Mooney, Will You Please Go Now.
These were followed by Bright and Early Board Books and Big Bright and Early Board Books which were abridged versions of the Bright and Early Books and included some Dr. Seuss titles.
There are many anniversary editions of Dr. Seuss books. Some of them include new material. For example, the 50th Anniversary Edition of The Grinch Who Stole Christmas includes 32 pages of commentary and archival images written and compiled by Seuss scholar Charles D. Cohen. It also includes three Seuss holiday offerings: "Perfect Present," a poem about the fluff-footed, frizzle-topped, three-fingered Zift; "The Hoobub and the Grinch," and "A Prayer for a Child."
Books by Dr. Seuss's wife, Helen Palmer
Helen Palmer, Ted Geisel's first wife, was an accomplished children's author. One of her books, A Fish Out of Water, was an expansion of Dr. Seuss's short story “Gustav the Goldfish.” It was illustrated by P. D. Eastman. Her best-known book is Do You Know What I'm Going To Do Next Saturday? (1963), and is illustrated with photographs by Lynn Fayman. She also wrote I Was Kissed by a Seal at the Zoo (1962) and Why I Built the Boogle House (1964).
Some of Dr. Seuss's books were included in Random House anthologies of Beginner's Books. For example, The Big Green Book of Beginner's Books (2009) and The Big Orange Book of Beginner's Books (2015) each include six Dr. Seuss books. My Big Book of Beginner's Books About Me (2011) includes three Dr. Seuss books and three books by other authors.
Books in collaboration with other writers
Dr. Seuss collaborated with Michael Frith in 1975 to write a book with the title, Because a Little Bug Went Ka-choo. Random House published the book under the pen name, Rosetta Stone. This is the only known collaboration published during Geisel's lifetime.
British and foreign language editions
British editions were similar to the American versions but made minor changes, most of them involving British spellings, for example substituting pyjamas for pajamas. Dr. Seuss books have been translated into 30 languages.
Books published posthumously
Daisy-Head Mayzie (1995) was the first Dr. Seuss book published after his death. The Bippolo Seed and Other Lost Stories (2011) contains seven stories published in Redbook from 1948-1959. What Pet Should I Get (2015) is from a manuscript discovered by Geisel's second wife, Audrey Stone, in 2013. Three coffee table books of Geisel's art have been published also. This is not a complete list and undoubtedly there are more books to come.
Collectible Dr. Seuss material does not end here. Early Seuss material can be found in issues of Dartmouth's humor publication, Jack-O-Lantern, sometimes under pseudonyms, and in issues of Judge, Life, Vanity Fair and other magazines. Art Buchwald's syndicated column of July 30, 1974, is the text of Marvin K. Mooney, Will You Please Go, but now with the name Richard Nixon substituted for Marvin K. Mooney. Seuss was the originator of this column when he sent a copy of the book, with Richard Nixon's name written in place of Marvin K. Mooney on every page, as a gift to Buchwald.
You can also collect promotional material like vintage Cat In the Hat buttons and vintage Dr. Seuss toys such as Grickily the Gractus, a snap-together toy manufactured by Revell in 1960. Finally, there are Dr. Seuss original artwork and reproductions. Chances of buying an original piece of art by Ted Geisel are slim because few made their way into the hands of private collectors. However, since his death, limited editions of some of his artwork have been made available by his estate. These are sold exclusively through authorized Dr. Seuss galleries. More information is available at drseussart.com.