I like this print. It does what it is intended, wasn't too hard to print. Not sure it needed the raft but I let it print that way just in case.
If you are looking for improvement ideas, I have a few.
I'm sure this has impact on the strength and stability of the components and their abililty to hold together heavy books. But, there are only a set number of ways the components can fit together. A few adjustments to the design would optimize the fit.
The small "m/f extender" could do with more ribs on the male side. It could also do with more depth and ribs on the female side. The angle shape of the corners in the small extender would need to be adjusted to fit closer inside the larger extender. I've added pictures below to visualize this opportunity.
Despite all that, I liked the print.