Why not "Mein Bücher."?
Wrong case. Plural needs 'die', feminine, which changes mein to meine. Mein Hund -> Meine Hunde, for example. A little more: http://en.wiktionary.org/wiki/mein#Inflection
What does "miene" mean? because I spelled it wrong and it said it right
Duo usually accepts misspellings if the number of noncorrect letters does not exceed 1. Miene means expression.