The brief is fairly simple - find the answers to the questions below and then go find the cache. However, as is often the way though the practicalities may not always be quite so easy!
The Intention
The idea behind this series of 10 caches is to create a series that combined two of my favourite pastimes (other than caching) - reading and clambering round at height (in this case in trees), and to cover all the 4.5 terrain points too with a progressively more difficult series of questions/clues, all of which are based on a 'classic' book. And when I say 'classic' I mean within popular boundaries (nothing too high brow here) - this will of course be subjective to the individual, but I hope you will agree all the books are in some way 'classics', whether well established or modern. ***The difficulty rating will be based only on the questions/book, as the terrain for each will be broadly similar, at 4.5, as each will involve a decent tree climb (not the one in the side pictures - that's just a random tree I have photographed) to get the physical cache.***
The first eight in the series can be done in any order you wish
The Challenge
There are eight books to work out, one for each cache, and two 'bonus' caches (more of that later). I have not given you the title and author of the books so you need to work out which classic book it is from the questions and then answer said questions to obtain the co-ordinates for the cache. Please note, the questions relate only to the book and not the film if one was made.
The Terrain and Cache
As mentioned above, each in the series is rated as 4.5 on the terrain side of things due to the fact that each cache is in a tree which will need climbing to get to the cache (because to claim the cache you need, or at least one person in your party needs, to actually get to the cache and sign the log, not just see the cache).
So here's the safety bit - just because I have placed the caches in the trees does not mean that that tree is safe to climb, only that I was happy to climb it when I placed it. If you arrive at GZ and you think it is not in your capabilities to get up and down, or you do not have the correct equipment (and/or not know how to use it!) then walk away - the decisions to climb (and therefore the consequences) are entirely your responsibility. Please also remember that branches can break or get weakened over time, rain or wet branches will make things slippery and winds will all increase the difficulty/danger.
Having said all of that I had great fun setting these caches up so I don't want to put you off at all - but you alone take responsibility for your own actions as I cannot take responsibility for them.
Right, that's the formalities over with! 
Onto the fun.
The Questions
This is the first with the easiest difficulty rating for the puzzle so hopefully this won't tax you too much. You may or may not regard this particular book as a classic yet but you can't deny it's popularity (hence it being my easy first choice).
N51 then:
- A = The numerical position in the alphabet of the last letter in the name of the character who met Harry off of the train for the first time.
- B = The number of letters in the first name of Harry's young female friend.
- C = How many faces did Professor Quirrell have?
- D = The first number of the platform at King’s Cross station where the Hogwart’s Express leaves from.
- E = How many players are in a Quidditch team?
W002 then:
- F = The number of school houses.
- G = How many Mirrors of Erisid are there?
- H = The position in the series of this book.
- I = Harry gets his first broomstick. The position in the alphabet of the second letter in it's name.
- J = The teacher Harry dislikes the most – how many letters in his surname?
So, a straightforward substitution in the formula N51 AB.CDE W002 FG.HIJ gives you the position of the cache.
The Bonus Caches
As mentioned above there are two bonus caches in this series (aren't you lucky!) The first (Clambering With the Classics #9) can be attempted when you have established the titles and authors of caches 1-8. Questions will be answered in the same way as this cache and the co-ordinates found. This attracts a D/T rating of 5/4.5 due to the work you need to do before it.
The final cache and 2nd bonus (Clambering With the Classics #10) attracts a cumulative 5/5 D/T rating as you will only be able to attempt this after solving the 9 caches before AND climbed all the trees because you will need to make a note of each tree that you have climbed. So good luck with that!!
You can validate your puzzle solution with certitude.
Don't forget to make a note of the tree when you climb it!