Since you are reading guidebooks, I would suggest two:

"An Hour from Paris," by Annabel Simms. Both St. Germain-en-Laye and Ecouen are favorites of mine. Others that have been mentioned above are in this book, with full details about how to get there.

For exploring around Paris, try "Quiet Corners of Paris," by Jean-Christophe Napias. I found the La Mouazia area in the 19th arr. to be like a day trip out of Paris without the travel.

Enjoy!