Here's a great way to remember adjective ending rules, from jess1camar1e in a discussion here:http://www.duolingo.com/comment/556140 --
-Big 3 get an -e (der, die, das) der alte Mann, das kleine Kind, die schöne Frau
-Changin' gets -en (plural and case changes) den alten Mann (accusative), der schönen Frau (dative), die kleinen Kinder (plural)
-No 'the'? Adjective takes over (no 'der' word or just an 'ein') Kaltes Wetter gefällt mir nicht (das Wetter). Ein guter Mann ist schwer zu finden (der Mann).
Note changin' means the definite article differs -- has changed -- from its nominative singular form (der, die, das). So feminine accusative, for example, doesn't get an -en: "Ich sehe die schöne Katze" -- I see the beautiful cat.
That's true in mixed inflection (e.g. after keine or ihre) and weak inflection (e.g. after die).
But here there is nothing before the adjective and so the adjective takes strong inflection.
Rule of thumb: in strong inflection, the last letter of the adjective is the last letter of what the definite article would be in that combination of gender, number, and case.
Bücher here is nominative plural and the definite article would be die which ends in E, so the adjective has to end in E as well (interessante) to show the gender, number and case -- since there is nothing in front of it which could do so.