MGMT employees are of course allowed to buy the books they need, but prior to buying you should consider if borrowing the book from the library would be sufficient. If the library has not got the title, you can suggest that the library purchases the book for you to borrow. See http://library.au.dk/en/borrowing-and-returning/suggest-a-title/
If you need books for teaching purposes, please always talk to the course responsible first in order to see if the books can be obtained from the publishers who are often known to supply free books for teaching purposes.
You need approval prior to any book purchase. Please see MGMT's purchase guidelines
BSS Books is also a good option: http://www.bssbooks.dk/en/home.asp or info@bssbooks.dk or +45 8615 5777.