The history of comic books/manga

was an artist of cheap illustrated books in Japan the original inspiration of Manga?

English: Portrait of Katsushika Hokusai by his disciple Keisai Eisen. from Wikipedia commons

The history of Manga discusses this in the beginning of the film.

what I found interesting was that one of the pioneers for pamphlets of drawings and watercolours of ordinary life was the same one who became famous as the artist behind the WAVE.

Katsushika Hokusai (Japanese, 1760–1849). Under the Wave off Kanagawa (Kanagawa oki nami ura), also known as The Great Wave, from the series Thirty-six Views of Mount Fuji (Fugaku sanjurokkei), ca. 1830–32. Japan, Edo period (1615–1868). Polychrome woodblock print; ink and color on paper, 

the Met Museum has an essay on the technical aspects of this iconic woodcut.

This is strikingly evident in the towering wave that breaks over the leftmost boat. When Eijudo's anonymous printing masters laid down the outlines of the design, they printed the dark vertical stripes first, using a mixture of Prussian blue and indigo to create a dark gunmetal blue. Then they printed the hollow of the wave, applying a pure Prussian blue over the initially printed stripes, and filling the white spaces left between them. The transition—from the deep blue, produced by the double printing, to the bright and saturated pure Prussian blue—animates the surface of the wave, adding visual depth and movement. This simple technique allows for a more suggestive, three-dimensional rendering of the wave and heightens the impact of the print.

ArtInContext article about the artist: and how western art and pigments influenced his art, but also how his woodblock art influenced the west. Many of his scetches are on that site

The British museum however notes his wood prints went beyond influencing high art:

The vast phenomenon of popular manga publishing in Tokyo, as we enter the Reiwa era (from 1 May 2019) of new Emperor Naruhito, is startlingly reminiscent of the vast phenomenon of popular print and illustrated books published in Edo in the late 1700s and early 1800s....

Between the ages of 16 and 19, Hokusai was training to cut the woodblocks used to print popular books and prints. He's famously supposed to have cut the last six pages of text for a novel about the Edo brothel district, Yoshiwara, published in 1775. From about the age of 20 he switched to being a print artist, and quite a few of his early designs of kabuki actors have survived.

 

The point is that Hokusai knew the popular print industry inside out – as a block cutter, author and artist. In the 1780s and 1790s he was regularly designing illustrations for the so-called 'yellow cover' or kibyōshi genre of popular comic fiction; and sometimes writing the stories for them too. In the Manga exhibition catalogue, scholar Adam Kern calls these 'comic books'...

Hokusai was an incredibly inventive artist, always trying different genres and subjects, sometimes creating new ones. In the early 1800s, he collaborated with the leading author of long adventure stories, Bakin, to develop the wildly popular genre of popular fiction known as yomihon (literally, 'books for reading'). Hokusai developed a new style of action-packed illustrations that were filled, often literally, with explosive drama.

In the 1810s, Hokusai was particularly busy producing 'picture manuals' (e-dehon), commercially printed drawing manuals full of pictures to copy, that spread his style widely in the general population throughout Japan, not just in Edo (Tokyo). The most famous of these, was Hokusai manga (15 volumes, 1814 – 1878).

The preface to volume one makes it clear that this title was suggested by Hokusai himself, a still unusual use of the word manga, which can playfully be translated as 'brush running away with itself'...

Katsushika Hokusai (1760-1849), Strange Tales of the Bow Moon, 1807.

again, if you are interested, go to the link and view the wonderful illustrations.