What results from the reaction of pent-3-en-2-ol with hydrogen bromide?
It is very rare for a reaction to have no side products, and this reaction is certainly not exceptional in that regard. I'm actually a bit surprised your book chose this reaction to illustrate the regiochemistry of $\ce{HBr}$ addition to an alkene, as this would compete with the formation of an allylic cation, which are generally more stable than alkyl carbocations. Being more stable, these cations will form more quickly:



giving the product you correctly assumed would also be formed. You probably wouldn't obtain this though, as it would continue to react, forming a dibromoalkane (as would the products listed by your book):