Are the following two compounds isomers or not? I feel they are enantiomers(non-superimposable mirror images) however my book states that they are not isomers? I cannot understand why?
You are right, these two molecules are enantiomers. If you rotate the bottom molecule 180 degrees, you can see that it is the mirror image of the top molecule. Enantiomers are stereoisomers, so the only reason your book would say they are not isomers is that it (unfortunately) contains a typo.