Channel assignment mechanisms in multi-channel wireless networks are often designed without accounting for adjacent-channel interference (ACI). To prevent such interference between different users in a network, guard-bands (GBs) are needed. Introducing GBs has a significant impact on spectrum efficiency. In this paper, we present a channel assignment mechanism that aims at maximizing the spectrum efficiency. More specifically, this mechanism attempts to minimize the amount of additional GB-related spectrum that is needed to accommodate a new link. Furthermore, inline with the IEEE 802.11n and the upcoming IEEE 802.11ac standards, our assignment mechanism supports channel bonding, and more generally, channel aggregation. We formulate the optimal ACI-aware channel assignment that maximizes the spectrum efficiency, while supporting channel bonding and aggregation, as a subset-sum problem. An exact exponential-time dynamic programming (DP) algorithm, a polynomial-time greedy heuristic, and an epsilon-approximation are proposed and compared. Results reveal a considerable improvement in spectrum efficiency achieved by the proposed algorithms compared to the recent work in the literature.