Instructions:You are given a small voucher giving you £5 off any book over the value of £10, if you buy anything in-store. So what you could do is buy something trivial like a chocolate bar, obtain the free voucher at the tills and then use it to get the discount during another transaction.
"This voucher enititles you to a £5 saving when you spend £10 on any book. Voucher is valid from Monday 16 March to Sunday 29 March only in WHSmith High Street stores. Excludes Outlet Stores, Books by WHSmith at Selfridges Stores, Online and Travel Stores including those at railway stations, motorway service stations, hospitals and work places. Offer applies to one book only. Subject to availability. Cannot be used in conjunction with any other promotional voucher. Only one voucher can be redeemed per transaction and it must be surrendered upon use. No cash alternative. Photocopies will not be accepted and the voucher is not transferable. WHSmith reserves the right to reject any voucher it deems, in its sole discretion, to have been forged, defaced or otherwise tampered with."
I've also noticed a BOGOF with best-selling books on their website. May be worth checking out.