There isn't much context to work with from the OP. I assume the moment of inertia it is talking about is for the cross-section of the blades which relates to how it deforms under load.
The material stiffness also affects how it deforms but geometrical solutions have a lot of potential to reduce the deformation while changing the material only affects it linearly. Besides, sometimes materials are already fixed because of other constraints such as oxidation, temperature resistance, price, manufacturability, availability, etc.
Lift produced by the blades will deform them and if the moment of inertia (and the material stiffness is not chosen to compensate) it could result in the blade having a significantly different profile than the one initially studied for the undeformed geometry. Therefore, the calculated lift will differ from the actual lift that it will produce if the deformation is not considered and it's big enough to have an impact.
Also, in the dynamic case, if the blades are too wobbly all kinds of bad things can happen to it.
Regarding the book
@OnlyPhysics was asking for, if the post is related to what I'm saying I'd recommend any book about Mechanics of Materials that talks about beams for a conceptual understanding of the matter. For example, Mechanics of Materials by Barry J. Goodno and James M. Gere.
For a more focused analysis of that particular problem, I don't know any book though. Since the profile of the blade is constantly changing along its length and its geometry is fairly complex I'd assume it must be solved with a combination of CFD and FEM.
