Smyth's book is a reference grammar; there's an electronic copy posted here on Textkit, so you can browse through before purchasing it, or even use the electronic copy instead of purchasing it. I think the suggestion is that you obtain an introductory textbook---an Introduction to Attic Greek if Aristotle is your goal. This type of book will be organized as a series of lessons, introducing grammatical information and vocabulary progressively, with exercises designed for each lesson, and short dictionary designed especially for that textbook. It might even provide a concise reference grammar. Perhaps somebody else could recommend a good Attic book---I think there is a recent thread regarding that issue.
Regarding the Septuagint: It is a peculiar Greek, being a rather literal translation from the Hebrew. Once you have finished an introductory Attic textbook, you could move easily to the LXX, noting the peculiarities as exceptions. Conybeare and Stock's "Selections from the Septuagint" explains those peculiarities and provides selctions with notes, but it assumes that you have studied Greek previously. It is also available for free on Textkit.
Here are two resources for LXX studies when you're ready:
http://ccat.sas.upenn.edu/gopher/text/r ... /lxxmorph/
This site provides grammatical analysis and the root for each word in the LXX.
This site provides some resources for LXX, including some short reading passages with notes.
Welcome to the forum. I wish I had discovered it earlier. It will save you some headaches for sure. Good luck!