All excellent places that are relatively off the radar for Anglo tourists, perhaps why you have difficulty finding them in guide books. That does not mean they are "not recommended" actually it's a good thing that they are not so well covered in guidebooks. In Spain these places are very well known, and traveled to by domestic tourism.
Valencia is my new favourite city in Spain, it's like how Barcelona was 20 years ago before mass tourism destroyed that city's historic centre. From Valencia you have more options, you can head south to explore La Albufera Wetlands National Park, where Paella originated from. You can also head north and explore the less touristy Costa del Azahar, Peñíscola is certainly worth a look, a Knights Templar crusader castle on a rock that juts out into the sea, surrounded by a whitewashed old town, very Levantine. Inland you find the ancient ruins of Saguntum, which Hannibal invaded to start the the Second Punic War with Rome.
Zaragoza as well is hardly known to Anglos I think, but is well regarded in Spain for its Moorish Palace and Roman ruins as well as the Catedral de Nuestra Señora del Pilar, one of the great churches of Spain. Zaragoza is one of the rare examples of Spain of a town touched by almost all the major civilizations that passed through Spain, much of which can still be seen like the extensive Roman ruins and Aljafería Moorish Palace. Excellent tapas and food scene too. I think that Zaragoza is one of those undiscovered gems that Rick calls "back doors". If you have extra time, there are also plenty of good day trips from Zaragoza too, like the Calatayud Wine Region, Loarre Castle, Olite royal palace, and the witches' village of Trasmoz.
Merida's city is maybe not the most charming but it has the best Roman ruins in Spain that can hold its own with most Roman ruins found in Italy. Merida is the capital often overlooked and underrated Extremadura region, in my opinion, one of Spain’s most fascinating regions. It is full of time-warped old towns, castles, monasteries, and beautiful countryside.
Tarragona is ancient seaside town with very good Roman ruins, including an amphitheater, intact aqueduct, and Praetorium Tower, was once the capital of Roman Hispania (modern-day Spain). Tarragona's winding medieval old town is a delight to explore too. Nearby is Les Ferreres Aqueduct, a Roman Aqueduct you can actually still walk on top of, a few km hike from Tarragona.
Any one of these towns are worth several nights.