Identical

Feelings, wo, wo wo, Feelings....

Compare:

"The plots of this book and that book are identical."
"The color of this book and that book is identical."
"The main characters of this book and that book are identical."
"The publisher of this book and that book is identical."

Gee, I don't know what I feel now. I'm off to Google this one.

OK, back. Relevant examples:

Was:

"...the vertical distribution of heat and particles was identical in megaplume 1..."
"...the number of runs and rewards was identical in both..."
"The definition of variables and outcomes was identical..."
"...the subtyping of gliomas and astrocytomas was identical in all 55 cases..."
"Evolution of starch and sugars was identical in the 4 ripening stages..."

Were:

"The vials of phenol and guanethidine were identical except for the names..."
"..."the original inhabitants and native rulers of the countries of Ethiopia and Egypt were identical..."
"Russia at the time was largely a medieval country, where the worlds of church and state were identical"
"During the night, the values of Tp and Tc were identical..."
"George Sicard's contention that the powers of orders and jurisdiction were identical..."

Obviously wrong:
"The number of peaks and pattern were identical in both cases..."
"...it became evident that the teaching of Hus and Wyclif were identical..."

I think the key is given by the noun that precedes the prepositional "of." If it is singular in form, then the verb is singular also ("was") and if plural, then it also agrees ("were").

OK, disregard all that as I did not really answer the question, did I.

As you used it, yes, "essence" should be singular, because it is a single essence shared by both things. "Color" is another good example. "The color of Mallory's hair and Jason's hair was identical."

But there are senses in which you would use "essence" and "color" in the plural, so: "The essences of Spanish lavender and English lavender are identical for the purposes of woud healing." ("Essences" meaning "essential oils.") The colors of her kitchen curtains and her table linens are identical." (More than one color being spoken of here.)

OK, have to stop thinking and start working now :)

Your problem is one of syntax. It would be better expressed "The essence of the two books is the same."

Joe, you're goddamn brilliant. "Your problem is one of syntax." What the heck did you think we were doing, mistaking it for a banana?

Not just for the sake of dissenting:

I'm inclined to believe that the sentence portion "the essence of this book and that book" refers to a single essence that both books share, in which case it might be grammatically correct to say it is identical (I don't think so. Can a single thing be identical all by itself and without having to be identical to something? I think it takes two to be identical.), but at the very best it is redundant, because you are saying that it is identical to itself.

"The essences of this book and that book were identical," on the other hand actually tells us something. Here we have two essences, and those essences are identical. Likewise, I would say that the sentence "the color of this book and that book is identical" is either grammatically incorrect or redundant.

In any case, if the phrase "the colors of this book and that book are identical" frightens you, you can always rephrase: "this book and that book have the same essence" or "this book and that book have identical essences" (more awkward).