Kindle library vs Nook Library?

In both libraries, you are most definite to find out most of the books you need. You have the B&N store with over 2.5 million titles and Amazon book store with over 1 million titles plus many titles you can find on the google search, making both pretty much equal in count.

Its not about the library, its just about the service provider I.e B&N and the Amazon. You are more likely to get better service from Amazon.

Kindle vs Nook - Discussion and details on features of both devices:

http://compare-e-readers.com/KindlePwNookLight

On Kindle and Nook you also have the option to borrow books from your nearest library (if it offers online books), you can find tons of free books also on both.

The benefit the Nook has is the microSD slot and the capability to read ePub format. Whereas on Kindle you will have storage space for over 1100 books and you can convert non-DRM protected ePub books to Kindle readable format.

Hope this helped!

I don't know if there is any quantitative or qualitative way to compare.

Are you asking about eh tablets or the ereaders? I will assume ereaders.

First - what do you mean by library?

Do you mean the ebooks you buy?

I think both have all of the bestsellers but it is my impression that Kinlde has more low cost and free new author and indie author books. I use this site to find what Kindle books are currently free at Amazon - I have no found a similar site for Nook.

http://www.dailyfreebooks.com/

Do you mean the Kindle Lending Library (the books you can borrow from Amazon is you are an $80/yr Kindle Prime member)? I do not know if Nook has a similar program. For both you can find unaffiliated groups of users who band together to loan each other books (Kindle owner to Kindle owner, Nook owner to Nook owner - loans are 1 time for 2 weeks).

Do you mean using Nook and Kindle to borrow ebooks from a local library via the Overdrive program? I haven't heard what if anything is different between when Kindle and Nook can borrow since the books are delivered to your device by the ebook store (Amazon for Kindles, Barnes&Noble for Nook)

https://search.overdrive.com/