I used Riccardo Latini’s A Santiago lungo la Via Della Plata e il Cammino Sanabrese, bought at a bargain price from the blessed Ivar. This is the only guide I know of that caters for both walkers and cyclists, and the text indicates the difficulties you may encounter on a bike - the mud and floods are seasonal hazards; the rough terrain is permanent - and proposes an alternative route where necessary. The information is clearly and concisely presented - no mystical paths or extraneous details to distract you - and the maps are easy to follow as you ride, so even if you can’t read Italian you can still see where you are (or should be) going. There is also a Spanish guide specifically for mountain bikers, La Via de la Plata en btt, but its odd format suggests it’s intended to be stashed in your back pocket rather than displayed in the map pocket of your bar bag. As an experienced road/touring cyclist, by the time you’ve ridden from San Sebastián, via Porto, Lisbon, Seville to Santiago, you should be well positioned to supply an English-language cycling guide of your own. Buen Camino bicigrina!