Just like babies crawl before they walk, authors usually write early books that are simpler than their later ones.
After all, creating a novel is tough. Until you get the hang of it, it can be hard to do the Proustian thing. Also, you might have a better chance of getting published -- and building an audience -- with a first book that's not too taxing to read.
Examples abound of authors who penned one or more straightforward novels before going on to write more layered books featuring a larger cast of characters, intricate plotting, time shifts, foreshadowing, and so on. J.K. Rowling and J.R.R. Tolkien certainly traveled that literary arc.
Tolkien's The Hobbit (1937) was delightful, but didn't have the depth of The Lord of the Rings trilogy (1954-55). Rowling's wonderful Harry Potter and the Philosopher's Stone (1997) was fairly linear, while several of her later books -- such as the gripping Harry Potter and the Deathly Hallows (2007) -- were multidimensional indeed.
Margaret Atwood wrote several elaborate (and superb) books in mid-career -- including The Robber Bride (1993) and The Blind Assassin (2000). But her novelistic efforts started with The Edible Woman (1969), a quirky book that satisfied readers without making them work too hard.
Margaret Drabble's third novel The Millstone (1965) is a no-frills story of a single woman who becomes pregnant. It's quite good, but Drabble's The Sea Lady (2006) is a mature tour de force in comparison.
Writer extraordinaire Cormac McCarthy never settled for average prose, but his first two novels -- The Orchard Keeper (1965) and Outer Dark (1968) -- were less lushly worded than later works such as Blood Meridian (1985), The Border Trilogy (1992/1994/1998), and No Country for Old Men (2005).
Dipping back into the 19th century, Charles Dickens matured from the humorous The Posthumous Papers of the Pickwick Club (1837) to the complex Bleak House (1853), Herman Melville advanced from the not that deep South Seas tale of Typee (1846) to the epic Moby-Dick (1851), and Emile Zola evolved from the relatively straightforward The Belly of Paris (1873) to the sprawling mining-themed masterpiece Germinal (1885).
Another way to make the literary crawling-to-walking transition involves writing short stories before moving on to novels, as did Junot Diaz and Jhumpa Lahiri. Their compelling short stories were far from simple, but a novel is obviously a more difficult undertaking.
Yet there are the rare cases of debut novels that end up being the most masterful book an author ever writes. Examples of this include The Heart Is a Lonely Hunter (1940), in which Carson McCullers skillfully juggles the interlocking stories of four disparate people; and The Sheltering Sky (1949), the haunting North African saga by Paul Bowles.
Can you name other authors who (like Bowles and McCullers) wrote mid-career-like novels on their first try? And can you name other authors who (like most of the writers mentioned in this post) didn't do ultra-layered work until after they had one or several novels under their belts?