Translation:I am going to buy manga in Ikebukuro.
Does anyone else feel like 漫画の本 is slightly redundant? 漫画 certainly refers to the genre, which I gather is why it was written this way, but it also refers to a given physical comic book, so this essentially saying "a comic book book".
PS please forgive me translating 漫画 as comic book, I realize it is its own thing, this is just for demonstration purposes.
The verb in your sentence and the verb in duolingo's translation are different.
池袋で漫画の本を買います。 (Ikebukuro de manga no hon o kaimasu)
I'm going to buy manga in Ikebukuro / I will buy manga in Ikebukuro / I buy manga in Ikebukuro.
漫画の本を買うために池袋に行きます。(manga no hon o kau tame ni ikebukuro ni ikimasu)
I'm going to go to Ikebukuro to buy manga / I will go to Ikebukuro to buy manga / I go to Ikebukuro to buy manga