When you do something twice a month, you do it bimonthly. Your bimonthly book club meeting will keep you busily reading to stay caught up.
Bimonthly is one of a group of confusing words (including biweekly and biannually) that have two meanings. You can use bimonthly to mean both "twice a month" and "every two months." The roots of the word are the Latin bi-, "twice" or "double," and monthly. The double definition means you might need to clarify how often your book club will actually meet.