Some classic problems appear in multiple textbooks. For example:
Some of the common topics covered in the solution manual for "Solid State Physics" by Ibach and Lüth include:
If stuck, use the solution manual only to check the first two lines of the derivation. This often provides the conceptual spark needed to finish the problem independently.
First, the harsh truth:
The best legitimate help comes from communities like Physics Forums (physicsforums.com) and ResearchGate. Here, individual problems are discussed in threads. While you won't find "Chapter 5, Problem 12" pre-solved, you will find rigorous mathematical derivations for the toughest questions regarding:
Calculating reciprocal lattices and Brillouin zones.
by showing how the transcendental equation leads to "forbidden" energy gaps. Explain why the "Effective Mass" ( m raised to the * power