Booking train travel in Spain online in advance

With the company that actually runs the train.

For most parts of Spain that will be Renfe, www.renfe.com. But their website is not very user friendly, so for the main high speed routes I recommend their competitor Iryo, www.iryo.eu.

I’ve always used Renfe.com to book train tickets in Spain but here’s an excellent website that will give you comprehensive information about train travel in Spain & elsewhere:
seat61.com/Spain

I booked my train tickets on renfe.com. In terms of cost the website of the company that runs the train is the most obvious and best site to buy from. I suppose that other train ticket resellers that have a website in English would be "easier" to use. I suggest doing whatever you can to figure out RENFE's website; try to avoid having to buy from another ticket reseller. Similarly, for bus tickets, try to figure out the website of ALSA before resorting to buying from any other bus ticket-reselling website.

I second the suggestion to have a look through the seat61 guide.

For researching options, I think trainline.com is worth using. As well as Renfe services it will also show services from the other high speed operators such as Ouigo and Iyro (but it doesn't include regional operators such as FGC).

Trainline's software is vastly better than Renfe's in working out connections. Not an issue, if you are just going between major cities on a direct route, but useful for many other trips.

Trainline also includes bus routes from the bigger coach operators such as Avanza and Alsa (and can work out inter-modal train+coach journeys).

Of course, having done your research, you don't actually have to buy from Trainline and can go to the operator's own site(s) to make the purchase. Trainline clearly states who the operator is, so it is easy to do this.

All the above might be entirely irrelevant if you are just planning a simple city to city journey where there is only one operator, of course! If you mention your plans, people could advise more specifically.