The oldest known illustrations of book are believed to be, Egyptian papyrus roll. These were preserved thanks to the sand in Egyptian deserts. It is known that in China around 13 or 14 century were made books out of wood, to be more specific they were carved into the wood.
We should not forget the Sumerian books that were made on clay tablets but there were no illustrations on them, and no prove has remain from Greek and Roman period. During the Byzantine period book illustrations are centered religious ideas.
After the emergence of new artistic directions and techniques in the 19 and 20 centuries, book illustrating is revolutionized and with the emergence of new technology, artists and designers take the illustrating to a new and higher level.
Image from page 576 of “D. Ioannis de Solorzano Pereira … Emblemata centum, regio politica : Aeneis laminis affabre caelata, vividisque, et limatis carminibus explicita, & singularibus commentariis affatim illutrata : quibus, quicquid ad regum instituti
“Decus in armis”
Bandits of the West (1953)