or other approved "bookstore" you will most likely be downloading an illegal, copywrited document. ACI sells its materials only and I've never seen them offer free downloads.
If you are consistently doing concrete design under ACI guidelines, why not simply buy the document? If you need it for a quick check on something for a one-time event, perhaps you could either ask specific questions on this forum, or call around to other engineers in your area to see if you can borrow it.
ACI has a CD version of their "Manual of Concrete Practice" that is fully searchable and you can print any sections you need. It is a great bargin, call ACI and become a member and you get a price break too!