Let's translate: you have items (books). Each item has a weight (price) and value (pages). Your knapsack capacity is (budget). get the highest value (pages). It's the same structure as 0/1 Knapsack! Each book is either bought or not.
You can't buy fractions. The constraints ( ≤ 1000, ≤ ) fit the standard DP approach. Recognizing disguised problems is a key skill. Many interview questions are classics in costume.