Categories concerns the general categories used in thought which include the essential/accidental distinctions, privation/possession, kinds of change (generation, destruction, increase, diminution, alteration, change of place) and the tendencies of objects/entities. It's hardly a page turner but his thoughts are not entirely systematic but they can be used like a system. A key book to understand his thoughts.
On Interpretation is concerned with linguistics and meaning. In sum, Aristotle sets to provide definitions (for what a name is and what a verb is, and then what a [16a1] negation, an affirmation, a statement and a sentence are.) And continue to discuss the logical relations they have with one another. I cannot precisely say whether the rules of logic differ between English and Ancient Greek but the sense is communicated rather well. Logic, I am sure, transcends language convention.
Highly recommended for any budding linguist.
Both read together will give one a good impression of the systematising and logical mind Aristotle has. If you manage to finish these and enjoyed them, then continue to read the other three in Aristotle Organon. (That's what I endeavour to do). Enjoy!