I have used or am using both of the below supplements with my son. They complement Thinking Through Science because they teach and reinforce the information in TTS.
The first I can recommend is Botany in 8 Lessons by Ellen Johnston McHenry. My son went through this course last year and he has such a good foundation in botany because of it. It isn't difficult reading either. He found it very enjoyable. You can buy it anywhere and it is not expensive. There are digital versions around too.
My second recommendation is The Zoology Coloring Book by Lawrence M. Elson. For those who don't like coloring, there isn't any coloring that isn't important. You can tell them the coloring is for labeling purposes only. For those who enjoy using colored pencils and detailing their art, there is a good amount of coloring for them.
Other than the coloring, though, there is a page of detailed information regarding all the major phyla in the animal kingdom with details about many major lifeforms.
I assign a half a page a day to read and detail. You could make it more or less. You can also be really organized and assign the pages of the zoology book that coincide with when each life form is introduced in TTS. ISBN-13: 978-0064603010 ISBN-10: 0064603016