Having lived most of my life in the American Southwest, I feel I can answer based on pure immersion and observation.
Question 1: I don't personally know of any resources where you can learn about Spanish influences, other than talking to people who live with both Spanish and English. My parents, my in-laws, and my wife all speak Spanish, and just through pure observation, you can see that Spanish has had a few influences on English.
For example: the words fiesta and siesta are commonly used without being translated. In Albuquerque, New Mexico, we have a famous gathering of hot air balloon enthusiasts called the Albuquerque Balloon Fiesta. This is considered normal usage of the word fiesta, even though strict English grammar would require that you translate fiesta into party or something similar. Fiesta sounds better to the locals, and it is the word of choice to describe this gathering.
In a similar manner, siesta, with its connotation deriving from Mexican siestas, can sometimes be viewed derogatorily as a lazy break or nap when one should be working. This derogatory connotation stems from the stereotype of the lazy Mexican. The lazy Mexican stereotype comes from Americans seeing Mexican people taking their siestas during the hottest part of the day (Northern Mexico has a lot of desert areas, and is closest to the American-Mexican border). Once the heat relented, the Mexicans would get up and continue their daily activities. But Americans saw this as laziness, because they view a work day as a monolithic chuck of time with minimal breaks. But the negative connotation with siesta stuck, and is still associated with the word to this day.
Words that have just been borrowed from Spanish include Mexican food names, such as taco and burrito, and words that express feelings, such as pronto and andale. Street names in the American Southwest are commonly named after people and places with Spanish names that the local English-only speakers just have to learn to pronounce (such as Juan Tabo or Montaño). Taco Bell is particularly guilty of slaughtering Spanish with food names like chalupa and quesarito. Even to this day, jalapeño is still pronounced ha-la-peen-yo, which comes close to the correct pronunciation, but still falls laughingly short.
Question 2: Spanglish is generally evil. No, I'm kidding. But it is my experience that most people who respect both English and Spanish hate Spanglish. The common refrain from people who hate Spanglish is "English or Spanish, choose one." With good reason, of course: grammatically speaking, no one should express themselves in two languages at the same time. But the truth is that people who live with multiple languages may have been introduced to new concepts in only one language. Subconciously, those concepts belong to those languages. So Spanglish is a side-effect of knowing certain concepts only in English or Spanish.
For example, there is a habit of Spanish speakers to end English sentences with the Spanish word mejor. This is actually a shortcut: you can express that it is better to do something in a particular way by using the word mejor at the end of your sentence. Most people understand what you mean. But short-cutting one language through another that particularly annoys fluent speakers of both languages. Yes, you may have to use more than one word in one of the languages, but at least you are not expressing yourself in multiple languages at the same time.
There is an even worse phenomenon in Northern New Mexico (note: not Northern Mexico) where English words are pronounced with Spanish rules and accents. Imagine pronouncing the English word brake with Spanish rules and accents: it would be the equivalent of an imaginary word in Spanish spelled braque. This is a generally reviled version of Spanglish outside of Northern New Mexico.
The best way to know about these things is to actually live and talk to the people here. The citizens of California, Arizona, New Mexico, and Texas are the best resources for finding out how Spanish has influenced American English, because they deal with both languages the most.
