TranslateBooksWithLLMs is a full-stack web application for translating eBooks (EPUB, DOCX, TXT, SRT) using multiple LLM providers (Ollama, OpenAI, Gemini, Mistral, DeepSeek, Poe, OpenRouter, NVIDIA NIM, LiteLLM). The architecture is a Flask server with Socket.IO for real-time progress streaming, a JavaScript SPA frontend with i18n support across 7 locales, and a SQLite-based checkpoint/resume system for interrupted translations. The core pipeline extracts content with HTML/XML placeholder preservation, chunks text using tiktoken-aware splitting, translates via configurable LLM providers with multi-phase fallback (LLM → token alignment → original), and reconstructs the output file. Additional features include glossary management with NER-based term suggestion, style extraction from source text, TTS generation via Edge-TTS and Chatterbox, and a comprehensive benchmark framework for evaluating translation quality across 35+ languages. The codebase demonstrates strong engineering with clean abstractions (adapter pattern for formats, factory pattern for LLM providers, generic orchestrator for translation pipeline), extensive testing (unit, integration, characterization with golden snapshots, e2e), and several genuinely reusable components. Notable patterns include a token-aware chunker with exact reassembly, an API key pool with round-robin rotation on 429 responses, an ordered concurrent executor that preserves chunk ordering, a retry manager with exponential backoff and circuit breaker, and a placeholder system for preserving HTML/XML structure during LLM translation. The benchmark subsystem and prompt optimization tool are supporting features that add research value but are secondary to the core application purpose.