You are definitely allowed click on ads on other hubbers' pages-it's all part of the adsense programme which enables the hubber and 'Hubpages' to earn money by each click.(Divided 60/40 between HP and hubber).
You are perfectly free to buy a book advertised on another hubber's hub.If it is advertised in their Amazon capsule ,they will earn commission when you purchase.If it is on a google ad ...there is no reason why not -,the writer will gain from the click only and not the purchase in that case.
It's an essential part of revenue creation for all.
Never ever though click on your own page ads -or never ask anyone to do it on your behalf ...that's a serious no-no and you risk losing your own right to use adsense.