docs: Add TTS engine quick reference guide

- Created user-friendly quick reference card
- Clear decision guide for choosing between engines
- Quick start instructions for both engines
- Feature comparison table
- Troubleshooting tips and pro tips
- Links to detailed documentation

Makes it easy for users to understand and choose the right engine.

TTS_ENGINE_QUICK_REFERENCE.md

@@ -0,0 +1,135 @@
+# TTS Engine Quick Reference
+
+## 🎯 Which Engine Should I Use?
+
+### Use Kokoro TTS (Default) if you want:
+- ✅ **Zero setup** - works immediately
+- ✅ **High quality** voices
+- ✅ **Many voices** - 54 voices, 9 languages
+- ✅ **Multiple languages** - English, French, Japanese, Spanish, etc.
+
+### Use Piper TTS if you want:
+- ✅ **Pitch control** - adjust voice pitch per character
+- ✅ **Fast synthesis** - quicker generation
+- ✅ **Lower memory** - smaller models
+- ⚠️ **Manual setup required** - one-time voice download
+
+---
+
+## 🚀 Quick Start
+
+### Kokoro TTS (No Setup)
+1. Start the app
+2. It just works! ✨
+
+### Piper TTS (3-Minute Setup)
+1. Install Piper: `cd backend && ./install_piper.sh`
+2. Download voices from: https://github.com/rhasspy/piper/releases/tag/v1.2.0
+3. Place `.onnx` and `.onnx.json` files in `backend/piper_models/`
+4. Done! See `backend/PIPER_SETUP.md` for details
+
+---
+
+## 🎛️ How to Switch Engines
+
+### In the UI:
+1. Go to **Voice Assignment** screen
+2. Click **"Kokoro TTS"** or **"Piper TTS"** button
+3. Voice dropdown updates automatically
+
+### Via API:
+```bash
+curl -X POST http://localhost:8000/api/tts/engines \
+  -H "Content-Type: application/json" \
+  -d '{"engine": "piper"}'
+```
+
+### Via Environment Variable:
+```bash
+# In backend/.env
+TTS_ENGINE=piper  # or kokoro
+```
+
+---
+
+## 🎵 Pitch Control (Piper Only)
+
+Adjust voice pitch per character in semitones:
+
+```
++12 = Very high (one octave up)
++6  = Child-like
++3  = Slightly higher (feminine)
+0   = Original voice
+-3  = Slightly lower (masculine)
+-6  = Deep voice
+-12 = Very deep (one octave down)
+```
+
+**Recommended:**
+- Female characters: +2 to +5
+- Male characters: -2 to -5
+- Children: +5 to +8
+
+---
+
+## 📊 Feature Comparison
+
+| Feature | Kokoro | Piper |
+|---------|--------|-------|
+| Setup | None | Manual |
+| Quality | ⭐⭐⭐⭐⭐ | ⭐⭐⭐⭐ |
+| Speed | Medium | Fast |
+| Voices | 54 | 5+ |
+| Languages | 9 | English |
+| Pitch Control | ❌ | ✅ |
+| Memory | 500MB | 10-50MB |
+
+---
+
+## 🔧 Troubleshooting
+
+### "Voice preview failed"
+- **Kokoro**: Check internet connection (first-time model download)
+- **Piper**: Download voice models manually (see PIPER_SETUP.md)
+
+### "Voice model not found"
+- Download `.onnx` and `.onnx.json` files
+- Place in `backend/piper_models/`
+- Restart backend server
+
+### "Piper not found"
+- Run `./install_piper.sh`
+- Or install from: https://github.com/rhasspy/piper/releases
+
+---
+
+## 📚 Documentation
+
+- **Piper Setup**: `backend/PIPER_SETUP.md`
+- **Piper Guide**: `backend/PIPER_TTS_GUIDE.md`
+- **Status**: `PIPER_STATUS.md`
+- **Full Details**: `PIPER_TTS_INTEGRATION.md`
+
+---
+
+## 💡 Pro Tips
+
+1. **Start with Kokoro** - Try it first, it's easier
+2. **Try Piper for variety** - Pitch control adds character depth
+3. **Mix engines** - Use Kokoro for most, Piper for special characters
+4. **Download multiple voices** - More variety in your audiobooks
+5. **Test voices** - Use preview button before generating full audiobook
+
+---
+
+## 🆘 Need Help?
+
+1. Check `backend/PIPER_SETUP.md` for setup
+2. Run `python test_piper_direct.py` to test Piper
+3. Check backend logs for detailed errors
+4. Fall back to Kokoro if Piper has issues
+
+---
+
+**Remember:** Both engines work great! Kokoro is easier, Piper is more advanced. Choose based on your needs. 🎉