Every gym (and closet) needs a skeleton. You can figure out how joints move, how muscles move those joints, and how anatomy affects exercise technique ... if you have one for reference. Half size ones are pretty good and are inexpensive. Full Size ones have better detail but can be pricey and can be unwieldy if you are trying to demonstrate how the body moves.
The quintessential anatomy reference, Gray's Anatomy, provided for FREE at Bartelby Great Books Online.
My Bodyweight and Age Adjusted Strength Standards can be had for free at this link. They can be printed out on 8.5x11" or A4 paper. Or you can click on the image below to get taken to a site that sells big poster versions of the standards and other posters I've created.
It's not the same as actually being able to touch and move a skeleton around, but it'll do in a pinch. The folks at 3D Toad have an interactive (you can spin it on its axis 360 degrees) and FREE online skeleton.
A FREE Anatomy & Physiology text provided courtesy of Bill & Melinda Gates and Openstax College.
But really, these are unique and good books that are useful resources in the real world of fitness delivery
THE ORIGIN OF THE CRITICISM
GET SOME FREE SCIENCE INSTRUCTION
If you want to get better at science you need to read science and have reference texts handy so you can look things up. These are the ones I use. For both the Guyton's Medical Physiology and Gilroy's Anatomy texts, you can buy older editions for cheaper. And if you don't want to spend any money then there are options for that as well ---->
A difficult but valuable read.
THE SUPPORT FOR CRITICISM