"thee" is the dative (and later accusative) inflected form of "thou", the second person singular pronoun.
In your sentence above "Read" is the imperative form of the verb. The subject is elided since it's implicitly second person (singular or plural) (as imperative is used to give orders to people you're talking to) and if it wasn't elided it would have been in nominative case. Middle english had an ending -(e)th to form a plural second person imperative.
"Learn the grammar of past and present languages, else you'll attract pedants from the intertubes"
In your sentence above "Read" is the imperative form of the verb. The subject is elided since it's implicitly second person (singular or plural) (as imperative is used to give orders to people you're talking to) and if it wasn't elided it would have been in nominative case. Middle english had an ending -(e)th to form a plural second person imperative.
"Learn the grammar of past and present languages, else you'll attract pedants from the intertubes"