If you would like please consider becoming a professional member $99/year of ACM then you also get access to Safaribooksonline. Personally I think its good value for money.
Last year, I explicitly checked with Safari customer service about this, told them this is too good to be true since a regular Safari membership costs 3x of the ACM membership. The customer service replied and confirmed there is nothing different. So go for it :) The basic ACM membership is really a good deal.
I have been a subscriber for 3 years, and can confirm it's the full SafariOnline subscription, on top of ACM Magazine (your choice or paper or electronic format), plus ACM DL Library.
Yes, it seems too good to be true. But it is real, and I am happy to support both services.
when I checked this out a few years ago it was a subset of you also get access to Safaribooksonline - about 600 books only. Maybe it has changed now....
https://www.acm.org/membership/membership-options