I might have missed something. If "I read eleven books a month." is another solution, could it be "en måned" instead of "måneden"?
'a month' translates to 'i måneden', as 'a month' doesn't mean 'one month', but 'per month'.
If I wanted to say "I read 11 books this month", should I use "Jeg leser elleve bøker i denne måneden"?
I'm assuming 'read' rhymes with 'red' in your example (past tense), but it would be "Jeg leste elleve bøker denne måneden" (no 'i').
I didn't mean past tense, but maybe "I'm reading" is better describing what I meant. Like saying you have started or continued 11 books at the same time.
Anyway, thanks :) I think my problem was in the "i" preposition.