How much has El Cid been romanticized after his death? Do historians believe that he was regarded as a Spanish hero and unifier at his death, or have his actions been exaggerated?
Spaniard here, hoping to shed some light on the heroic part (and apologies for my English, not firs language):
We have a written account of the Cid's life in the form of an epic poem, the Cantar de Mio Cid, which also happens to be the very first literary work in the Spanish language. The consensus is that it started as a poem/song to be performed in front of an audience so the exact date of its composition is unknown. The oldest manuscript that we have is from circa 1320 and on it the copier mentions that he works with a book from 1207 by a certain Per Abbat- he says that "Per Abbat le escrivió" which in ancient spanish means "he copied" instead of "le fizo" or "le compuso" which would mean "he wrote" so the consensus is that the poem itself is older. Exactly how older, we don't know, for as I say it originated in the oral tradition and it was customary for the performers to change and adapt the poems, so although the version that we have can't be from before mid-XII century (geographical and legal references help set the time frame), the original could be older.
To summarize, it tells how the Cid falls from grace due to false accusations, regains his honour thanks to his courage in battle, falls from grace again when their daughters are repudiated by their husbands and regains his honour again thanks to the mediation of the king.
It does depict the Cid as a warrior of extraordinary skill and bravery and a faithful subject to his king but I wouldn't say he's depicted as a hero in the modern sense, for all over the poem we're reminded that his first and uttermost motivation is to best serve the interest of the crown.
Source: I had to read and study it in high school (ancient Spanish is painful to read but intelligible to speakers of modern Spanish). I'm afraid I don't know any English books but if interested I guess you should be able to find a translation of the absolute classic in the subject: "Cantar de mío Cid: Texto, gramática y vocabulario" written by Ramon Menéndez Pidal in 1908.
At Internet Archive, https://archive.org/search.php?query=cantar%20de%20mio%20cid%20AND%20mediatype%3Atexts will give you the poem in Spanish or French. The English is here https://archive.org/details/thelayofthecid06088gut Menéndez Pidal is great but I couldn't find a free copy.
Roderigo's devotion to the crown makes him a more modern hero, I would say, than so many other epic heroes obsessed with their own glory. Yeah, Roland, get everyone in the rearguard killed just so you can say to didn't summon help.