Answer by taking a quote from the following article:

Theodor Seuss Geisel ( ( listen); March 2, 1904 - September 24, 1991) was an American author, political cartoonist, poet, animator, book publisher, and artist, best known for authoring more than 60 children's books under the pen name Doctor Seuss (abbreviated Dr. Seuss) (). His work includes several of the most popular children's books of all time, selling over 600 million copies and being translated into more than 20 languages by the time of his death. Geisel adopted the name "Dr. Seuss" as an undergraduate at Dartmouth College and a graduate student at the University of Oxford. He left Oxford in 1927 to begin his career as an illustrator and cartoonist for Vanity Fair, Life, and various other publications.

Geisel's early artwork often employed the shaded texture of pencil drawings or watercolors, but in his children's books of the postwar period, he generally made use of a starker medium--pen and ink--normally using just black, white, and one or two colors. His later books, such as The Lorax, used more colors.  Geisel's style was unique - his figures are often "rounded" and somewhat droopy. This is true, for instance, of the faces of the Grinch and the Cat in the Hat. Almost all his buildings and machinery were devoid of straight lines when they were drawn, even when he was representing real objects. For example, If I Ran the Circus shows a droopy hoisting crane and a droopy steam calliope.  Geisel evidently enjoyed drawing architecturally elaborate objects. His endlessly varied but never rectilinear palaces, ramps, platforms, and free-standing stairways are among his most evocative creations. Geisel also drew complex imaginary machines, such as the Audio-Telly-O-Tally-O-Count, from Dr. Seuss's Sleep Book, or the "most peculiar machine" of Sylvester McMonkey McBean in The Sneetches. Geisel also liked drawing outlandish arrangements of feathers or fur: for example, the 500th hat of Bartholomew Cubbins, the tail of Gertrude McFuzz, and the pet for girls who like to brush and comb, in One Fish, Two Fish, Red Fish, Blue Fish.  Geisel's illustrations often convey motion vividly. He was fond of a sort of "voila" gesture in which the hand flips outward and the fingers spread slightly backward with the thumb up. This motion is done by Ish in One Fish, Two Fish, Red Fish, Blue Fish when he creates fish (who perform the gesture with their fins), in the introduction of the various acts of If I Ran the Circus, and in the introduction of the "Little Cats" in The Cat in the Hat Comes Back. He was also fond of drawing hands with interlocked fingers, making it look as though his characters were twiddling their thumbs.  Geisel also follows the cartoon tradition of showing motion with lines, like in the sweeping lines that accompany Sneelock's final dive in If I Ran the Circus. Cartoon lines are also used to illustrate the action of the senses--sight, smell, and hearing--in The Big Brag, and lines even illustrate "thought", as in the moment when the Grinch conceives his awful plan to ruin Christmas.

What else did he do in these books?
Almost all his buildings and machinery were devoid of straight lines when they were drawn,