The minimum required space between the stacks for wheelchair access is 36 inches (see illustration in ADAAG). The space that is preferred – and this word makes the difference -- is 42 inches. The guidelines are somewhat awkwardly worded in this regard, and so the illustration and the text are both needed to make it clear. See the ADA Accessibility Guidelines (ADAAG). Shelf height in stack areas is unrestricted.
You can contact the U.S. Access Board with any questions -- http://www.access-board.gov/contact-us