Major sources and notes
The following is a compilation of the sources that have been the most valuable to me over the past year and change. This page is intended both to give credit where credit is due and to provide you with a jumping-off point for other resources that may be of interest or use to you. Sources are grouped by general category and are accompanied by notes indicating how I used them and how you may want to use them.
HIgh school textbooks
Realidades Series
Boyles, Peggy Palo. Realidades 1. Upper Saddle River, NJ: Pearson Prentice Hall, 2004. Print.
Boyles, Peggy Palo. Realidades 2. Upper Saddle River, NJ: Pearson Prentice Hall, n.d. Print.
Boyles, Peggy Palo. Realidades 3. Upper Saddle River, NJ: Pearson Prentice Hall, n.d. Print.
This series is used by Severn School for levels 1-3; beyond that, the school uses Encuentros Maravillosos (a personal favorite that I unfortunately lost) and a set of AP practice books that I threw away as soon as I could. Anyway, the series is decent, and I listed it here mainly because I relied on it to coordinate my content with the classroom experience of a typical high school student. There are some things I wish the books would handle differently, and the organization is pretty confusing, but there are plenty of exercises, vocabulary lists, and review materials. However, if you are looking for a reference book, none of these are it.
Español Esencial Series
Levy, Stephen L. Español Esencial 2: Fundamentals of Spanish. New York, New York: Amsco School Publications, Ind., 2004. Print.
Levy, Stephen L. Español Esencial 3. New York, New York: Amsco School Publications, 2004. Print.
These books will always have a special place in my heart. This was the series I used when I was in middle school and learning Spanish independently. This collection is much more grammar-driven than Realidades, and the books are combinations of reference material and workbook sections, which I think is a pretty solid model and one that is very easy to use. If you are looking for a way to practice grammar, I would certainly suggest this set of books. Obviously, begin with the first one if you have no experience. I just lost my copy, so I couldn't use it for reference and didn't cite it.
Spanish-English Dictionaries (online)
English to French, Italian, German & Spanish Dictionary - WordReference.com. WordReference, n.d. Web. 03 July 2015. <http://www.wordreference.com/>.
SpanishDict. N.p., n.d. Web. 03 July 2015. <http://www.spanishdict.com/>.
Honestly, it is a toss up for me between these two dictionaries. I seem to prefer the actual dictionary part of WordReference for some reason, but SpanishDict has a very well-done conjugation tool that gives you a clear list of all of the conjugations for all of the tenses and moods of a verb. It also has a very good forum/discussion section for conversations about the finer points of Spanish grammar. Pick whichever you like. Whatever you do, DO NOT use Google Translate. Just don't. It is incorrect, awkward, and plagiaristic. Pick a dictionary -- a dictionary, not a translation machine -- and stick with it. I also recommend avoiding paper copies of the dictionary. Yes, they carry great nostalgia, but no, they are not practical, and they can be extremely frsutrating when you are trying to read or write something and want an answer in less than five minutes. Personally, I think they are a waste of money.
Grammar reference (print)
Butt, John, and Carmen Benjamin. A New Reference Grammar of Modern Spanish. 5th ed. London: E. Arnold, 1988. Print.
This book is an investment -- I believe mine was $40, but I got it as a gift, so don't quote me on that -- but it is utterly worth it. It is a pretty unique find: an English-language manual to Spanish grammar, and a remarkably well-executed one at that. If you have a question, there is a very, very good chance that you will find the answer somewhere in this book. It is not bedtime reading by any stretch of the imagination, and it is not intended to be something that you work through like a textbook or workbook, but if you are serious about Spanish, see if you can find a copy. Even an old edition from Amazon or Ebay will do the trick. This thing is invaluable.