Book Shop - State Design

Define $dp[j]$ = maximum pages achievable with budget $j$. This is the space-improved 1D (one-dimensional) version. For each book with price $p$ and pages $v$: if we buy it, $dp[j]$ = max($dp[j]$, $dp[j-p]$ + $v$) for all $j$ ≥ $p$.

Process books one by one. Iterate budget in reverse ($j$ from $x$ down to $p$) to avoid using the same book twice. The state tracks both money spent and which books you have considered. Two dimensions are needed.