I agree with John that it's better for you to continue with Pali Primer (PP). That's the book I started with too. I appreciate the gradual learning curve a lot. My only complaint is the choice of vocabulary. It includes certain words that are extremely rare in the scriptures; some of them exist only in the very much later texts (e.g. bhuupaala). Nonetheless, considering what we have now for English-speakers, it's still the best book to start with.
After PP, you may proceed to Ven. Narada's course. There's much more vocabulary and grammatical items here than in PP, some of which you would have gone through. You can flip through the pages, and pick those items that you haven't learnt. To my knowledge, there's answer key to only part of the exercises, which I think you may already have. (Anyone, do correct me if I'm wrong.)
When you're done with these two books you're quite equipped to compare the Pali and English translations of *certain* scriptural texts with the help of a dictionary. If you like, you can go to the actual scriptures, which is what I did. (I'm currently going through the Paatimokkha with the help of my teacher.)
Alternatively, you can move on to Gair & Karunatillake's book, which uses only real Pali sentences from the scriptures as well in its exercises. Although, as John pointed out, there's quite a heavy load to each lesson, the knowledge that you've gain from the two earlier books should make it much easier. Be forewarn that there's no answer key given (except an incomplete and "unofficial" one made available in our Files section).
In any case, you'd probably find some satisfaction being able to understand some of the texts directly in Pali. You may also then begin to find that some of your hypotheses about Pali are incorrect. The two books, being primers, necessarily generalize a lot and very often don't give you the complete picture. Reading the real texts will give you a better feel of Pali.
If you choose to go for the Texts itself, do stick to prose and stay away from verses for the time being. The Khandakas of the Vinaya Pitaka (if you're interested), such as the Vatta Khandaka, are quite straight forward; so are the Jataka stories (a sample of which can be found at the end of Ven. Narada's book). Do get Buddhadatta's Concise Pali-English Dictionary too. I find that where it differs from PTS's PED, it's usually more accurate.