Spanish Pronouns and Prepositions
I purchased this book from the Practice Makes Perfect series at the recommendation of other learners. Wish I had gotten it sooner! It would have saved me a lot of frustration in the mid level sections of the skill tree. Duolingo launches us into longer sentences with additional phrases, pronouns and prepositions without any instruction as to word order and other rules that are comunicated very clearly in this book!
I couldn't imagine truly using Duolingo without finding backup resources, or trying to learn French here without already knowing Spanish. Duolingo is providing a framework of repetitive exercises and hoping you get the grammatical theory intuitively, but if you don't just google it or get a basic grammar book like you mentioned!