Peter, at Streetwise Spanish, recommends the following resources to help you with your understanding of the Spanish language.
Peter says that you should be able to get them from your local bookstore. If not, just go to Amazon and buy them online. Please note that Peter is not receiving any fees for these recommendations and neither am I.
Spanish Verb Tenses, by Dorothy Devney Richmond.
This book really helps to get the various verb tenses nailed. Each tense is fully explained in an easy-to-understand style and the exercises are a challenge but enjoyable.
Complete Spanish Grammar, by Gilda Nissenberg.
This is from the same Practice Makes Perfect series. It is good for those of you who struggle to make sense of Spanish grammar - pronouns, reflexives and the impersonal for example. All the topics are clearly explained and the exercises will help you get to grips with Spanish grammar.
Correct Your Spanish Blunders by Jean Yates
Peter's third recommendation is a book that will help you avoid the common errors that mark you out as a typical guiri! Peter thinks this is a really useful book, he likes the way the errors are highlighted in a red box explaining fully how the mistake is made with examples of corrections that are nice and easy to follow. A very practical little guide which he says you'll use frequently.The other books which we have found useful are the Collins Easy Learning series which you will find in many local bookshops. I think we have most of them now. The one we mostly dip into though is Spanish Grammar which explains a lot of the things we struggle with on our Spanish course. The other one which we find useful is Spanish Verbs which helps us make sense of the irregular ones.
As for online resources, I find the dictionary at WordReference.com invaluable.