Based on my audit of your Ruby codebase, I see several refactoring opportunities that would not only improve the current system but also ease the eventual migration to Elixir.

The most pressing refactoring need lies in the error handling system. Currently, you have overlapping implementations across ErrorHandler, ValidationFramework, and within the CommandRunner class. These should be unified into a consistent approach that separates error creation, enrichment, and handling. This refactoring would directly map to Elixir's tagged tuple error handling pattern later.

Your builder classes contain significant duplication in how they manage external tool invocations. Extracting a common BuilderToolchain module would centralize tool verification and command execution patterns. This would also prepare for Elixir's more explicit function parameter passing rather than relying on instance variables.

The configuration system is quite sophisticated but tightly coupled to many components. Refactoring to use a dependency injection approach where configuration is explicitly passed to components would make the code more testable and prepare for Elixir's immutable data structures and explicit function arguments.

Your validation framework would benefit from implementing a formal strategy pattern to standardize how validators are selected and applied. This aligns with Elixir's protocol-based polymorphism and would simplify the eventual migration.

For the CommandRunner utility, the detection of command contexts could be extracted into a separate command categorization service. This separation of concerns would prepare for Elixir's functional composition approach.

The duplication in the epub_builder.rb file (which contains two identical class definitions) should be immediately addressed. This type of duplication creates maintenance headaches and confusion about which implementation is actually being used.

Your metadata handling appears to be embedded in various components rather than centralized. Creating a dedicated MetadataService would provide a single source of truth for book information and prepare for Elixir's more explicit data transformation approach.

Most importantly, before migration, I'd recommend increasing the test coverage across the codebase, particularly focusing on integration tests for the build pipeline. These tests will serve as a vital reference and safety net during the migration to Elixir.