Of course. I keep a book and as I paid for them, they are going to be used as I need them. The PO got paid for them they should and do honour them in my experience..
If they say !st or 2nd then they are still valid as first/second class stamp. If there is a price on it then it's value for the purpose of paying postage is the price printed on it
It;s annoying as stamps in books donl;t have a price on and so you know they are going to get stocks in at the current price and then add the rise on after they go up
All stamps since decimalisation are valid. As factor30 says, if they have a price on (eg 24p) then that is what they are worth. If they say 1st or 2nd you can use them as that. If you have any stamps with 1/2p on them then you can add two together to make another penny, eg 2 X 12 1/2p stamps put on a letter make 25p.
Dothawkes31: "It;s annoying as stamps in books donl;t have a price on and so you know they are going to get stocks in at the current price and then add the rise on after they go up"
Why is that annoying? Nothing to stop you doing the same and buying a surplus miles in advance if it annoys you. If they sell stamps at the correct price at the date you go to buy them I fail to see what odds it makes to you what price the shop bought them at?