I brought my father (69 years old at that time) up a four pitch route on Gorro Frigio, just above the monastery, one of the tallest Gorros for the first rock climb of his life. The descent was a via ferrata. I don't think there are many places in the world better than Montserrat for easy multi pitch.
My father had wobbly knees at the time so the funicular between the monastery and the Golos above was quite handy.
The route we did was very clean and quite... hmm... don't know if “sustained” is the right word, but it had no section of ledge-shuffling. I no clear idea which route we did as I just climbed what looked like the easiest way up that side of the tower, but there where plenty of bolted belays to choose from.
When planning where to go I was using
http://blogdelguillem.blogspot.fr/search/label/Aa%29%20Montserrat%20Central , a blog by two Catalan climber of quite advanced age. They have plenty of topos for many of the towers, and it should be enough if you're after really easy stuff.
As for guidebooks: remember that there are more than 5000 pitches of climbing in the Montserrat massive. There is no single guidebook to everything.
Depending on season, you climb either on the north or the south side. For single pitch routes, get “Montserrat Cara Sur, Vias Cortas, volumen 1” (Alfonso) for the South side, and “Montserrat Vertiente Norte” (Alfonso/Buxo) for the North side. The latter book also have some multipitch routes, but perhaps nothing I'd bring up a beginner on.
Montserrat is a great place to climb!
