http://tariff.businesslink.gov.uk/tarif ... untryCode=
I used to arrange customs clearance for UK import goods.
Books and other printed material whether or not of a single page are Duty free - I think VAT free too. however your book although a parcel would probably have been customs cleared as part of a bulk clearance (several packages at once often a whole load from the US because it's cheaper to do 1 entry than lots of little ones as parcel force will be charged by customs per entry) In this instance your book may not have cleared if there is another part of the package held for some other reason (if package contains any animal products like wool, fur or skin or any fruit, veg or wood - all the previous can be detained by port health and it takes a little while to arrange to get the offending parcel separated from the rest to allow them to be removed from customs bond.
There could also have been illegal goods or explosive goods (fireworks, seatbelt pretensioners etc) or even ball bearings which can be held up because they come under the same customs commodity code as those used for making ammunition (shotgun shells)
There are a lot of reasons it could be delayed fromo the above or something as simple as customs wanting to check the paperwork first (some entries are selected for document checks at random) these are the most likely reasons for a customs delay if you are not expected to pay VAT / Duty. However you may be charged if Parcel force used a basic catch all commodity code which they have been known to do. Any questions on imports or Customs let me know as I work for an international logistics company at the moment.