I had a real difficult time understanding this sentence from Pliny. It is used as an example in the Greenough grammar to eludicdate 'quisque' but it's actually 'quō' that is giving me trouble. It seems to me that 'quō' here is definition 2 in the OLD. which is:
~ = quō
2 (in correlative sentences, with comparative) ~ ‥ eo, hoc, etc., In proportion as (more, less, etc.) ‥ by that degree (more, less, etc.); (also without correlative adverb in main clause).
The examples they give are too hard to understand. I tracked the example from Ceasar down but to me the translation does not elucidate well the meaning of 'quo':
Caesar iis civitatibus, quae ad eius amicitiam accesserant, quod minor erat frumenti copia, pecus imperabat; calones ad longinquiores civitates dimittebat; ipse praesentem inopiam, quibus poterat subsidiis, tutabatur.
I should also add that the OLD in that sentence replaces 'quod' with 'quo' but I got the sentence from PHI. This is translated by Peskett as:
The supply of corn being too small, Caesar began to requisition cattle from the states which had gone over to his side, sent sutlers to the more distant communities, and himself endeavoured by all possible resources to meet the present want.
It seems that the translator used a sentence which if translated into Latin would not use 'quō'. Anyway, back to Pliny, it seems that it is the word order that is giving me the most order, so to rearrange the words into something an anglophone would find natural I have:
maior quisque bonus liber est quō melior
translated literally this would be:
[the] larger a good book is the better.
So here, 'quō' would be the second 'the' in a 'the ADJ-er x is, the ADJ-er' construction where both adjectives are in the comparitive. Also, here 'quisque' seems to refer to a hypothetical book whereas 'quidem' would refer to an actual book just not identified by the speaker.
Let me know if this is right.