If you decide to invest in a dictionary, it is better to invest in a good one that explains the usages of words rather than providing a one-word direct translation. For example, in the back of your text, when you look in the English section of the dictionary for the word "on", the definition you will find is "en". This refers to the prepositional meaning "to be on, in or even at something". It doesn't refer to the state of something that is "turned on", like the lights or a radio.
Your text book glossary tries to give you the most common usages, but it lacks explanations for the wide variety of meanings. Your best defense against misusing a term is to do the Double-Check: look it up in English and then look up the Spanish definition in Spanish.
Collins Spanish Unabridged Dictionary, 9th Edition ISBN: 9780061808258. A good desk reference dictionary will offer explanations of word use and have sample sentences to give you context. For example, when you look up the English word "run" in the textbook glossary, you find the Spanish word "correr". Can you use correr to describe a "run" of good luck, how to "run" a business or a "run" in a stocking? A good dictionary explains this and also notes when a word has different meanings in different countries.
Some good pocket dictionaries are the Pocket Oxford Spanish Dictionary Oxford University Press; 4th edition & the Collins Spanish Concise Dictionary, 5e
An excellent vocabulary reference book to keep in your home or office is The New Oxford Picture Dictionary by Jayme Adelson-Goldstein, Norma Shapiro; Oxford University Press, USA; 2 edition. This collection of illustrations presents detailed scenes in which all of the elements are identified in both English and Spanish.
A good, short grammar book is English Grammar for Students of Spanish: a Study Guide for Those Learning Spanish by Emily Spinelli Olivia & Hill Press; 6th edition.
For those of you who want in depth grammar explanations, a terrific reference manual is A New Reference Grammar of Modern Spanish by John Butt and Carmen Benjamin, McGraw-Hill; 4th edition
For help with verbs, 501 Spanish Verbs, Barron's Educational Series; 6th edition shows the conjugation of the 501 most commonly used verbs. This can be useful when dealing with irregular verbs. Now it includes a CD ROM with practice activities and a conjugation wheel.