It's so cool to hear you're writing a book!
Misconceptions? Oh, there were several.
Even though I did very thorough research upon deciding to get a little budgie of my own, there will always be some inevitable things you learn solely based on personal experience.
Gender identification was a big one--although I knew males had blue ceres and females had brown, younger birds don't exhibit these characteristics. I assumed that the "no young bird can be sexed correctly" misconception was true, and because of the pale blue on my little hen's cere, assumed she was male for the longest time.
After even more research and the revelations brought on by her first moult, I determined exactly how correct you could be when sexing young birds--which, in fact, can be quite accurate.
The second misconception I believed for a while is that as long as any food said "for Parakeets", it was a good food for parakeets to live off of. Now that I know better, I think it was just the lack of information out there about what domesticated birds really need to live long and healthy lives. Yes, it is true, the wild varieties out on the Australian outback to live mostly on seeds, but then, they are mostly nomadic and fly long distances each day, two things which radically change the diet expectations for domesticated budgies.
After I became concerned over my little girl's weight--because although slight, she still ate a lot of seed and I suspected liver problems--I discovered the benefits which come with a more wholesome diet of pellets, which allows for more nutrients and less concentrated fat.
Now that I know the truth about budgie diets, it has helped me to see things a little more critically instead of believing all that people may claim, be it on a package of seed or in the bird department of a pet store.
Hope this helped! Good luck on your book