[Something] is to be [past participle of a verb]
This is a grammatically correct sentence.
You can see this idiom in the well-know expression:
He is a force to be reckoned with.
In your example, the positive form might help you:
This book is to be labelled as an example of pseudointellectual postmodern rhetoric.
Which would mean "This book should be labelled as [...]".
In a more general sense, to say that Something is to be [past participle of a verb], for example This person is to be blamed for his actions means that people, in general, should blame him for his actions.
In your case
Even though this book is not easy to read, it shouldn't, in any way, be labelled as an example of pseudointellectual postmodern rhetoric.
Though, in this case, I find your sentence to be very clumsy and would find a sentence with should, along the lines of the one I presented, more appropriate and better-sounding.