Organising bookshelves

Lovely comments on Twitter last night from Alexander McCall Smith about ways to arrange bookshelves at home followed by Radio 4 Today programme picking up the topic and a variety of tweets this morning.
I once worked for an organisation where, for some inexplicable reason, I found myself involved with a variety of things including 2 libraries. Both libraries belonged to the same organisation but followed different Dewey classification systems and each had their own way of cataloguing and, well, doing everything. I do like the fact that, in a library, you can look at the catalogue and then easily find the range of books that you are looking for in the same place. Having said that, I recently spent some time in a local history library where a very helpful member of staff kept bringing me books that she thought I might be interested in but which she knew were not with the others I had been looking at.
But at  home what is the best thing to do? I have friends who follow the alphabet for fiction and then group non-fction in alphabetical themes - cooking, cricket, rugby, travel...... I used to follow the system known as - "Mum knows where everything is" - and it worked very well. Have we got a copy of "To Kill a Mocking Bird"? Yes - it is on the second shelf down to the left of the sofa, near some Garrison Keillor. Do we have any Nick Hornby? Try the small front bedroom, bottom shelf. I always knew where to find my own favourite authors - Anne Tyler, Carol Shields, E Annie Proulx, Margaret Atwood.... - hardbacks on middle shelf of first bookshelf by the door in the sitting room, paperbacks top shelf, left hand side of low shelf in my bedroom. I could usually remember where to find most things but there was no system apart from the pile of unread paperbacks either on the top of the middle bookshelf in the sitting room or in the pile steadily rising between the first and second set of shelves in the front room - those were the "to read" books.
The situation reached a point where there could have been some resolution earlier in the year when I decided to invest in 3 large new sets of bookshelves for the hall. This could mean that books could all be in one, or at most 2 places - hall and sitting room. Six large bookcases would accommodate everything in some kind of order and the shelves in bedrooms, landing, ktichen and dining room could be put to a different use. Old paperbacks could be sent along to the charity shops and everywhere would be tidy.Mmmmmm.
The new bookshelves are lovely. They were made to fit into two alcoves and they fit well. They are sturdy and attractive. There are no books that I would not want to read or re read on them. There is some semblance of order in the shelves: 2 shelves of books yet to read, one that I will and one that I should; one shelf of "amusing" books to dip into; some devoted to children's and classics; one each  on genealogy, cross stitch ( a weakness each time I go into our local Oxfam book shop), education, History and so forth. A couple of shelves of my favourties at the moment - Richard Russo and various authors writing about small town America....... and all the shelves are full.
How did that happen? How did the extra shelves become full and yet the other shelves around the house still seem to be filled with books? Even overfilled as books are still lying on top of others, rows still lie along the back of desks and still pile on top of units. I do still know where everything is but, and this is the great advantage in having bought the new shelves and done some moving around, in addition, I know now that we have a shelf of dictionaries and "word" type books which makes more sense than the old system and that most of the travel guides are in one place.
Nowadays I can also wonder who it was who bought a Russian dictionary and why we have travel guides to places I don't think any of the family have been to. I can look clearly at the number of books that I know I have still to read yet it does not stop me buying more. Will Kindles, laptops, i phones etc. stop me buying physical books? I very much doubt it. Do I have a system for my shelves? Sort of but it is still "In the mind of Mum" and it works.