The standard lisp approach is to define your API first, and write code the meets the API. You define your API in a manner that makes the solution to the problem you're trying to solve obvious. So behind this API, you can change your data representation however you please -- just don't break the API.
As for your specific question, which representation of a book is the simplest solution? That's the one you should be using. Hickey's talk on simplicity really does a good job of explaining this point.
The funny thing is that most of these APIs wind up with what's essentially a "this" pointer at the front of every argument list so you're really doing OO anyway.
I'd like to read a persuasive critique of OO but so far I haven't found one. Time objects are a really bad example because time calculations are exactly the kind of hairy mess you want to hide behind some kind of abstract API.
The funny thing is that most of these APIs wind up with what's essentially a "this" pointer at the front of every argument list so you're really doing OO anyway.
Do you have much experience with non-OO programming? Or even OOP, for that matter? How can one honestly believe that every instance of passing data into functions is automatically OOP? Did you even consider the fact that this has nothing to do with objects?
Time objects are a really bad example because time calculations are exactly the kind of hairy mess you want to hide behind some kind of abstract API.
You miss his point here. Time not being an object would not imply that it can't be abstracted into an API. Just see Clojure's clj-time library for an example.
I'm sure that GP is referring to point number 2 of the article -- which explicitly says that time should (or at least could) just be data with no associated methods.
I winced at the naïveté in that section myself. It sure seems like he's arguing that OO programmers muddy the waters when they make something as "simple" as time and dates into an object with methods for storage and retrieval.
What we do in LedgerSMB usually is write our API's first in SQL which is decidedly not OO.
Then we write objects then flexibly encapsulate that.
Then we write UI and automation logic around that.
Now, long-run we're trying to find better ways to encasulate the behavior inside SQL. I don't think we've settled on a method yet but may do so sometime between 1.5 and 2.0 (a year or three).
Would an API be defined as "A set of functions that define the available operations common to that set?" If so, would those functions then delegate to progressively more concrete functions that work against data structures to perform their duties? I suppose it's obvious where I'm going here. I don't see how this is any different from just saying the word "Object."
Don't get me wrong, I'm not trying to come off too sarcastic here. I'm actually quite interested in challenging the OO norm that is obviously too easily accepted right now.
As to coming up with the "simplest" representation, that's almost certainly excellent advice whether doing OO or FP. It should be noted that Hickey has a rather specific notion of "simple" that may be rather different from more typical intuitions. This is why Hickey had to differentiate between "simple" and "easy", where "simple" means not intertwined with other things, NOT what seems like the least amount of work. The least amount of work is what is "easy", not what is "simple".
If one is encapsulating the representation behind an API, this simplicity principle is much less important for the representation than it would otherwise be, however, as any complexity of the representation is fire-walled at the API. And the Law of Demeter is a good principle for helping to keep an API "simple", as functions of the API that follow The Law of Demeter should return only data and not objects.
I'm still not sure how this API business meshes with Hickey's advice, however. He seems to be rather adamant that hiding data behind an API is the wrong thing 90% of the time. I have no feel, however, for knowing if something as complex as a book falls into the 90% case or the 10% case.
Rich Hickey specifically says that his approach makes you more readily adaptable to future requirement changes, rather than less adaptable, as an OO die-hard might claim. I'm not sure I understand how the "get it right to begin with" approach fits into the adaptabile-to-future-requirement-changes promise.
I see how following OO, as long as you also follow the Law of Demeter, could live up to its promise of helping you adapt to future requirement changes. It has that unfortunate cost, however of requiring you to sometimes write a zillion little delegation methods upfront, which could be quite a combinatorial chore if you don't have the Demeter system automating this for you.
Also, I'm pretty sure from listening to a number of Rich Hickey's talks, that he is saying very specifically NOT to hide your data behind an API. Encapsulating the data behind an API would just be the same as the OO approach for all intents and purposes. Hickey is saying to let the data be data. I.e., just data.
As for your specific question, which representation of a book is the simplest solution? That's the one you should be using. Hickey's talk on simplicity really does a good job of explaining this point.