FEUP's Library gives you access to books in print and digital format (ebooks). Printed books are located on the library's shelves (check out the subject organization in each library's floor). The access to ebooks is provided through many different subscribed collections, some of which are multidisciplinary and other are specialized in engineering areas.
Ebooks' collections allow you to access thousands of titles on a 24x7 basis.
If you are outside of FEUP's campus, you just need to activate a VPN (Virtual Private Connection) to get access.
For some collections, to access the books offline, you may need to previously install the Adobe Digital Editions, like for example in Ebook Central.