Re-reading a page makes it familiar, and familiarity is easily mistaken for understanding. The test is whether you can produce the idea without the page.
The trap
Close the book and say it back
Pulling an idea out of memory is what strengthens it. Putting it in again does much less than it feels like it does.
The method
Three ideas beat three hundred pages
A book you can summarise in three sentences you will still have next year. One you merely finished is already gone.