It will always be optimal for all books that Marko reads (the whole book) to be adjacent. Why? Let us presume they are not adjacent. Then there would exist a place where Marko read book ~i~ and have read the covers of book ~i+1~. He will definitely not reduce his inspiration if he read book ~i+1~ instead of book ~i~.

From that, we can conclude that the solution will always have first a few books that Marko has read only covers of, and then a few that he has read whole. If he skipped first ~x~ books, he can maximally read next ~y = \lfloor\frac{n - b \times x}{a}\rfloor~ books. To determine his inspiration we can use prefix sums. We can do this for every possible ~x~. Total time complexity: ~\mathcal{O}(n)~.