Does the current Spain RS guide include Merida?

No, it does not, you can go here: https://store.ricksteves.com/shop/p/spain-guidebook and see what it does cover.

Many like Lonely Planet, I find them up and down depending on the area, they cover many areas, but somewhat lightly, I think they lost their luster in the past decade or so.

I do like Rough Guides, sort of targeted for the budget traveler.

Might want to head to your local bookstore or library to browse. I like to look at guides, and still buy some (Kindle version), but most of my planning is done from blogs and websites, and hotels and restaurants always come from the web (or once there).

If you're doing both cities in 4 nights, I am assuming that your time in Merida will be quick.

We're doing a day trip there from our stay in Caceres next year. I just planned the day trip out using an online search for sites in Merida and some discussions on this RS site. It was pretty simple - for a visit that short, I just included the highlights - the Roman Forum, the Roman Art National Museum, and Casa del Mitreo roman villa remains. All sites are grouped together nicely and they are an easy walk from the train station. On that walking route, we also get to walk by the bullring, just to see it, as we're not going in.

Thanks Paul. I honestly never think of Rough Guides, but should. I did go to our local bookstores and they have Lonely Planet, RS and Moon Guide. :-/ Amazon obviously has more, but you can't flip through it.

I still like to have a comprehensive place that I don't need to evaluate every item, you know? I use websites more than books but it's nice to be able to go to the index, find what I want and read it in a voice I've already evaluated. Then I've got a framework for the sites that come up.

We're planning two nights in Seville and two in Merida, so I'll have a deeper look at your thoughts, Valerie, thanks. Definitely interested in the Roman sites!

We spent two amazing days in Merida in March. There are more than 10 Roman sites and you can buy a 48 hour ticket that gives you admission to them all.

We enjoyed the Roman theatre and amphitheatre the most, spending around two hours there. Casa del Mitreo was also good for the mosaics and the 792m long Roman bridge was exceptional. The Milagros aqueduct is also really good and in a lovely park on the river. There was just so much to see that we were very busy for our two days and we would love to go back.

We had one of the best meals we had in Spain at Sybarit Gastroshop which is right near the Trajan arch. We stayed at Hotel Ilunion Merida Palace which was right on the main town square.

We LOVED Merida!

What kind of Spain guidebook wouldn't have Merida? Actually, this doesn't surprise me -- back in 2014, before my Spain trip that year, I also discovered that the RS guidebook did not mention (at least not in the index) the quaint, medieval town of Zafra, which I visited after my Merida stay. (I don't know whether it's been included since then -- anyway, the RS guidebook I consulted then was luckily a library copy, not a purchase.)
But I also want to add to a couple of preceding comments, and say that I too loved Merida -- I think it's a good choice, especially if one can spend more time there than you propose. Anyway, for places like Merida, Zafra, and others that lie off the most heavily trodden tourist trails, you probably need guidebooks like Lonely Planet or Rough Guides. (In Spain and Latin America, my usual destinations, I've found Lonely Planet fully satisfactory.)

Thanks Faedus. I really like the written "voice" of the RS team, but I've come to terms with the fact that the country guides just aren't comprehensive. I have Lonely Planet Spain on kindle (ok, but I still want more), and will also check out a Rough Guide if I can lay hands on one. Appreciate your endorsement of Merida. I also found reference to Zafra interesting, and will explore.