All are English. "of" is often omitted, as is "of the". They're a given with "Both". That is, you can pick it up from the context.a) Both books/Both the books (?) /Both of the books you lent me are interesting.
Both of the books
Both the books
To place in the topic position, i.e., at the front of the sentence. The verb's object, along with its RC, has been topicalized:b) Both books you lent me I found interesting? (correct or clumsy?)
“the object has been topicalized” what’s the meaning of “topicalized”?
EX: I found both (of the) books (that) you lent me (to be) interesting.
"Normally" isn't clear. "to be" is there underlying; it's just not overtly realized. Again, this is another example of a form that can be picked up from the context. "to be" joins "interesting" with its subject "both books"; e.g., both books are interesting. I found them to be intersting. I found them interesting.c) I found both books you lent me to be interesting.
(do we normally insert the "to be" if the descriptive word does not immediately follow the noun it describes)
It's English. And if you tell us why you're asking, we might be able to guide you further.d) I found both books you lent me interesting.
"appropriate" isn't clear. "evolve" works in that context.e) Neither you nor your boyfriend wants to see the relationship evolve. (is the word "evolve" appropriate here?)