Due to the limited di?racting ability of millimeter wave (mm-wave) signals, solving the blockage problem in 60GHz WLANs is one of major challenges. Since multiple access points (AP) are typically employed in the indoor environment to ensure the coverage and to meet the increasing demand for extremely high data rate applications, in this paper, we propose a multi-AP architecture with which a MAC layer device called an Access Controller (AC) is employed to enable each station to associate and cooperate with multiple APs. In this way, multi-AP diversity can be exploited to solve the blockage problem. Since APs may not fully obtain the state of channels, we formulate the AP selection problem as a Partially Observed Markov Decision Process (POMDP) and obtain an optimal policy. Moreover, a threshold-based policy with reduced complexity is developed with which the decision for AP selection depends only on the number of consecutive transmission failures. The optimal threshold value for AP selection is also derived. Simulation results show that the threshold-based policy achieves almost the same performance as the optimal policy derived from POMDP.