Find the first book in the library. Note down the class mark, and the number of pages.
Take the number in the class mark, and divide by 26. The remainder is the letter of the section for the next book.
Next, take the subsection, and convert the letters to numbers. Take the units digit of each number - this is the class number for the next book.
Now take the number of pages, and convert each digit to a letter. This is the subsection.
Find the nearest book to this class mark, and repeat. See where you end up, and what path you take.
As an example, here's the route I took at Sussex:
The first book is AA 100 Cha, with 767 pages (a list of Law papers). This gives me V 381 Gfg as the next book to find.
The nearest book to this has classmark V 385.67 Pli, and 89 numbered pages (what appears to be letters and information about the navy during Churchill's time). This gives U 629 Hi.
Nearest to that was U 646 Old with 175 pages. This led me to classmark U 524 Age, almost on the same shelf.
The closest book was actually U 520.L1 Tho with 249 pages. This gave me a class mark of Z 085 Bdi.
That didn't exist, but what did was Z 103 Kah, a 1181-page cryptography monster. This would give me a code of Y 118 Aha, but I left the library at that point.
Have a go, and post your own route. Try from the last book as well, or for more randomness alternate between multiple collections!