The Invisible Fran is the 3rd book in the Franny K. Stein series. I got this book because I'm a huge fan of the invisible genre and basically read anything where someone turns invisible but after reading this I will definitely check out the rest of, if this book is anything to go by probably best junior fiction series written in a long time!
Franny is fascinated by science and is in fact a practicing mad scientist herself. She has a lab in her parents' house and is helped by her assistant the reluctant Igor, her pet dog. During a school class the teacher assigns the class to do a talk on their hobbies. Fran listens to the other kids' hobbies such as stamp collecting, baking and Irish folk dancing and is convinced that once she demonstrates her hobby of being a mad scientist that the others will abandon their boring hobbies and emulate her. She brings a two headed robot to class but the class isn't impressed. Enraged and pondering how to convince the class she notices Igor being swallowed by a previous experiment a giant chameleon (which is apparently is a common occurrence). That's it she decides being invisible is the answer to her problem so combines chameleon DNA, cellophane molecules and disappearing ink into a drink which she consumes. Now in real life you'd think she'd die wouldn't you, but in junior fiction it works and she's invisible. Her plan is to become the voice inside the heads of her classmates by whispering in their ears making them think they want to be mad scientists.
A very funny, fast paced entertaining story. The length is fairly short so you'll probably want to get a few of if not the whole series. They seem to be a lot cheaper buying as a box set which seems to retail for about the same price as just two Franny K. Stein books.
If invisible man type fiction is also what you're interested in also check out The Invisible Man's Socks, My Best Friend Is Invisible (Goosebumps) by R. L. Stine, You Are Invisible: CYOA #48 by Susan Saunders, The Invisible Day by Marthe Jocelyn, Frankenstein Makes a Sandwich by Adam Rex and you can even get an illustrated version of H.G. Wells 1897 classic The Invisible Man (Great Illustrated Classics).
If the reader you are buying for can read at an adult level (or if you are an adult and you want a great invisible book yourself) the best of this genre are the masterpieces Memoirs of an Invisible Man by H. F. Saint and Smoke by Donald E Westlake.