Pocket Por and Para: The only book you'll ever need!
Demystifying Spanish Grammar: Advanced Spanish Grammar, Clarifying the Written Accents, Ser / Estar, Para / Por, Imperfect / Preterite, & the Dreaded Spanish Subjunctive
A Frequency Dictionary of Spanish: Core Vocabulary for Learners (Routledge Frequency Dictionaries) (English and Spanish Edition)
Demystifying the Spanish Subjunctive: Feel the Fear and 'Subjunctive' Anyway
Easy Spanish Step-By-Step