Wikipedia has many excellent entries on regional dialects. I’m wary of taking all its information at face value, but the site provides good links to relevant sociolinguistics texts that would be tricky for laypeople to track down otherwise.
That being said, the site has some peculiar un-sourced dialect articles that leave me both scratching my head and yearning for more. One of my favorites is a little artitle entitled New Mexican Spanish. Its section on phonology is particularly notable. For instance, apparently this variety of Spanish renders pronounces /s/ as [h] at the beginning and middle of words so that…
Also, /b/ before /w/ apparently becomes [g], so that abuelo sounds like aguelo. And /n/ is sometimes inserted before “ch” so that muchos becomes munchos.
I can’t verify whether this information reflects contemporary use, as the article cites few sources. Which is a shame, because New Mexican Spanish has a rich and important history, dating back to Spanish settlement in the area around Santa Fe, Taos, and Los Alamos. I tracked down a century-old text via Google Books on New Mexican Spanish (there are some JSTOR articles, but I don’t have access) which suggests a variety quite different than the kind of Spanish we learn in school.
For instance, according to the author, this dialect would have had a rounded front vowel for /ue/ words like “bueno” (bu̯œno) and “muerto” (mu̯œrto). It also would have had Russian-esque vowel reduction, so that “tirar” would have been tɪɾaɾ, “de nada” would have presumably been something like də nadɐ, and “unidos” would have been along the lines of ʊnidʊs. (If my extrapolations of early IPA are correct).
New Mexican Spanish is, in fact, one of the oldest native dialects in the United States, preceding most of the more established English varieties. Which is why it’s a shame that one of the most recent articles I could find about the topic is titled New Mexican Spanish: Demise of the Earliest European Variety in the United States. Is this dialect still going strong?