I do not believe it is wrong to call a self help book a bible. It would be the same as saying it is all you will ever need to know about an earthly topic, written in one book. The only problem I see is this world and the things in it change daily. I could not imagine anyone being able to say that they could write a book that is complete and all you would ever need on a subject of any earthly thing.
Calling a tax self help book a bible is one of the books I would think least likey to have all the information you will ever need on taxes, because tax laws change daily.
Thank God, His word never changes and Jesus and God stay the same everyday, and the book they have inspired man to write is complete and all we could ever need to live by. And because it is inspired, it is as if God wrote it Himself and we can count on the information to never change, and the only book that you can call a Bible.
As far as calling a tax self help book a bible, or complete, it could only be complete at the time of the writing, and would become incomplete after they change the first tax law.
The only thing wrong with calling any self help book a bible, would be , if you thought it was all you ever needed on the topic. Because the way the world works, it will keep changing the answers so you will need to keep buying books to keep up with the questions.