Could publishing a book on a subject prior to a PhD reflect negatively on me later if it has errors?
Anything you write places you at risk of being wrong somehow. Certainly a book of this kind is asking for a wider audience that then might be aware of those mistakes. Alexandros points out in a comment that you certainly need to do your due diligence now to assure that what you write is as correct as it can be. No one is going to blame you if you suggest a response to a certain legal framework which is overhauled in three years but if you describe a chemical solution that could never react in the way you claim that one is on you. So how certain you can be that your work will remain correct/useful is somewhat field dependent. 

The big question is about this hurting your ability to start and pursue a PhD after writing such a book. I don't think it would. Except in the rather extreme case of your book becoming well enough known and wrong/controversial enough to annoy members of a PhD admissions committee. For the most part I would suggest that already being an active member of a practical field (read: has published work discussing the field) would be a very helpful piece of an application for a PhD program.