As a very long time member of the ACM, I’ve learned there are a few perks that come with it. The best, in my opinion, is the Learning Center, which provides ACM members with complimentary access to a number of useful resources, including the full content at O’Reilly Learning (formerly, Safari Books).
For the price of an ACM membership (currently $99) you can get access a resource that normally costs 5x that (currently $499).