I really liked the book Introduction to Protein Structure by Branden and Tooze. I am not sure it's in print anymore but your local library may have a copy. The PDF may also be available online.
Structural biology concerns all biological macromolecules and thus includes, for example, nucleic acids.
A nice definition is given at the MIT web pages : (https://biology.mit.edu/research/structural_biology)
Structural biology seeks to provide a complete and coherent picture of biological phenomena at the molecular and atomic level. The goals of structural biology include developing a comprehensive understanding of the molecular shapes and forms embraced by biological macromolecules and extending this knowledge to understand how different molecular architectures are used to perform the chemical reactions that are central to life...
And an example of aims and scope:
" The Journal of Structural Biology publishes papers dealing with the structural analysis of living material at every level of organization by all methods that lead to an understanding of biological function in terms of molecular and supermolecular structure"...
Dear Nicolas I think the foundations of structural Biology by LEONARD J. BANASZAK is a good reference for basic structural biology .please see book finder web.
Since by the time you came to ask that question , you probably have had enough basic biochemistry, physics and math. Thus, I recommed jumping into solving the macromolecuar stucture with texts like Gale Rhodes "Crystallography Made Crystal Clear" or Bernard Rupp "Biomolecular Crystallography .. "
Crystallography is much easier to understand if you " follow the math" . Thus, linear algebra, Baysian probability , Bragg's Law and Fourier transform are the foundations to understand the process.