Books are much harder to obtain in Guatemala than in the United States or Europe. We advise you to bring along three small books which will help you.
A very small Spanish-English dictionary that will fit in your back pocket and you can carry with you at all times, or a Spanish-English dictionary for your smart phone. If it is for your smart phone, spend the money to get dictionary that is offline: entirely on the phone. The first time I traveled with my smart phone the dictionary I had required an Internet connection and was therefore useless in most circumstances.
A small Spanish grammar. Your classes will be in Spanish so it will be helpful to be able to review the grammar you have studied in class afterwards in your native language. Hands down favorite because it is small but covers everything completely: Spanish Verbs and Essentials of Grammer by Ira Ramboz. It comes in three different covers, the yellow being the old. Price: 19 cents to $10.
A pocket-seized book of phrases for travelers. Armed with this and your dictionary (to substitute words for your specific situation) you should be able to get through most basic travel situations.
You should be able to get all three books for under $25.00. Travel light, don't bring any more Spanish books than these. If you don't bring them it is unlikely you will find ones as good in Guatemala. There are no bookstores in San Pedro.