Reddit mentions of The White Mountains

The White Mountains is exactly what you're looking for. I read it at around 9-10 years old, and for me, it was a good intro to sci-fi. The world building is plain and simple enough for younger readers to put together without too many questions, and the story itself is quite straightforward- there aren't any major "whoa, here's a surprise that's going to fuck with your head!" types of plot twists. It's definitely grounded in the adventure genre, with just enough mystery solving done by the main protagonists to make it accessible to younger readers.

The Tripods Trilogy is set in a future agrarian society where the human race has been conquered by aliens. We've been made docile and ignorant through the use of alien technology and the banning of knowledge. It's Young Adult fiction, but that's precisely what you've been reading so far.

In order: The White Mountains, The City of Gold and Lead, and The Pool of Fire. A fourth book, When the Tripods Came, was written later and is the story of when the aliens arrived (read it last).

The first two books were made into a BBC TV show in the 1980s. A movie is supposed to be coming out this year, The Tripods. All I know is that they changed the design of the alien ships because the book version looks just like Spielberg's ships in his War of the Worlds remake.

Summary from Amazon:
>Long ago, the Tripods--huge, three-legged machines--descended upon Earth and took control. Now people unquestioningly accept the Tripods' power. They have no control over their thoughts or their lives. But for a brief time in each person's life--in childhood--he is not a slave. For Will, his time of freedom is about to end--unless he can escape to the White Mountains, where the possibility of freedom still exists. The Tripods trilogy follows the adventures of Will and his cohorts, as they try to evade the Tripods and mainainn their freedom and ultimately do battle against them.