Currently half price at £9.99, but if purchased with another book priced at 1p or more you can us the '£5 off any book voucher' thus making it £4.99 plus whatever you pay for your other book.
See here http://www.hotukdeals.com/item/21094...f-vouchers-fo/
for a link to the pdf file with the 4 x printable vouchers, all valid until 31st Dec 2008 or hyperlink above.
or directly here http://whs.emreact.com/aem/clients/WHC001/vouchers/pdf/WHS0769FourVouchers.pdf
Please note: This offer is INSTORE only - book available online for £9.99, but the £5 off voucher is only valid for use instore
[Extra note: This deal was originally posted months ago, I can't find it anywhere on here so full credit to whoever the OP was, just its that time of the year when we are all pressie hunting and I'm sure there are many out there that don't know about this great offer! If a mod can find the original post, please delete this one, ta X]
p.s. edited original title as just realised you have to spend £10 not £9.99 to qualify using the £5 off voucher - those clever little dickies at WH Smith have realised and not made the book £10 this time!