For the sense of smell we had two great books.
This one if relatively basic most of the way through but it does introduce the olfactory bulb and sending messages from senses to the brain. Lamb was a bit weirded out as the olfactory bulb in the picture looks like a slater!
Lamb loved this book and we have had many conversations about allergies since. It probably goes into too much detail regarding synapses for a three and a half year old but you can skip that. It contains images from under a microscope of all of the allergens it discusses.
Both are available at Amazon Books.
We used the second as the basis for what we filled our squares with on our 5 senses chart.
Lamb drew pictures of flowers as the book discusses pollen as an allergen, we did little dots for pepper, he drew a dust mite, I drew a mushroom for mould as it's a fungi while Lamb insisted on attempting to draw the mildew as it appears in the book and we stuck a guinea fowl feather on as the book discusses down in pillows as an allergen.