pages of books

Let the total number of pages be $x$

Sophia finished reading $\dfrac{2}{3}x$ pages, and she has $\dfrac{1}{3}x$ pages that she has yet to read.

The question stated that what she read is $90$ pages more than what she hasn't read. This gives:

$\dfrac{2}{3}x - \dfrac{1}{3}x = 90\\ \dfrac{1}{3}x = 90\\ x=90(3) = 270$

Sophia's book is $\boxed{270}$ pages long

Let the number of pages she read be $x$ and the number or pages she has to read be $y$ .
$\begin{aligned} \dfrac{2}{3}(x + y) & = x\\ 2x + 2y & = 3x\\ x - 2y & = 0 \longrightarrow \boxed{1}\\ \\ x & = y + 90\\ x - y = 90 \longrightarrow \boxed{2}\\ \\ \text{Subtracting}\boxed{1}\text{from}\boxed{2}:-\\ y & = 90\\ \\ \text{Substituting y = 90 in}\boxed{2}:-\\ x - 90 & = 90\\ x & = 90+ 90\\ x & = 180\\ \\ \therefore \text{No. of pages} & = x + y\\ & = 180 + 90\\ & = \color{#69047E}{\boxed{270}} \end{aligned}$