"Neha buys a book from the shop."
Translation:नेहा दुकान से किताब ख़रीदती है ।
In the sentence 'नेहा दुकान से किताब ख़रीदती है', किताब is a direct object of the verb ख़रीदना just like in the English sentence. By separating किताब and the verb as in your sentence, it can no longer function as such.
You can make your sentence grammatically valid by placing a postposition after किताब: 'नेहा किताब को दुकान से ख़रीदती है'
Same as the original sentence (though you would use the definite article 'the' with 'book' compared to the original where you can use 'a book' or 'the book'). The main difference is what you are emphasising.
नेहा दुकान से किताब ख़रीदती है । is the answer to the question of what Neha buys from the shop. नेहा किताब को दुकान से ख़रीदती है is the answer to the question of where Neha buys her book from.
My mind is a little blown from this usage of को, in contradistinction to the typical translation of को as "to" that I've been exposed to hitherto.
"Neha book to shop from buys is."
Is को often used in this way to specify the object of a verb that occurs later in a sentence? Is there an easier way to think of it other than "to"?
नेहा दुकान से एक किताब ख़रीदती है is not wrong. However, एक is not required.
Hindi uses the indefinite article एक much more sparingly than English. The absence of an article can either indicate definiteness or indefiniteness in most situations. It is never wrong to include it but it is required only in a handful of situations like when introducing a brand new subject into the conversation.