"...seeing as it is a German name, it should really be pronounced “Soice” - the way Geisel said it himself! Ted eventually switched this anglicized pronunciation himself, since most people already said it that way and it “evoked a figure advantageous for an author of children’s books to be associated with - Mother Goose. Source info: [link]
Geisel's pen name is regularly pronounced /ˈsjuːs/ sewss, an anglicized pronunciation inconsistent with his German surname. He himself noted that it rhymed with "voice" (his own pronunciation being /ˈsɔɪs/ soyss) and Alexander Liang (his collaborator on the Dartmouth Jack-O-Lantern) wrote of him:
You’re wrong as the deuce And you shouldn’t rejoice If you’re calling him Seuss. He pronounces it Soice.
Geisel switched to the anglicized pronunciation because it "evoked a figure advantageous for an author of children’s books to be associated with—Mother Goose" and because most people used this pronunciation.
For books that Geisel wrote and others illustrated, he used the pen name "Theo LeSieg" ("LeSieg" is "Geisel" spelled backward).