linked from Paul Taylor's collecting site. --- Caring for Your Collectible Books
There's some good advice in the above blog article.
Collectible or not, I am a big believer in not storing thick books (e.g. 500+ pgs) upright. I hate to see the lower edge of the text block transform into that unsightly shallow 'U'-shape that weakens the spine and dirties the paper edge. This is a widening problem with new books, too, with publishers skimping on the robust spine needed to support the weight of even modestly thick and/or heavy blocks.
The reasoning behind leaving open space behind shelved books is probably practical as well as preservative. Some suggest that the air circulation keeps moisture traveling through the wall from settling in the book pages, but I would assume that the vapor barrier in modern houses would obviate this. I mainly do it because it makes it easier to remove books from the shelf without placing much direct pressure on the top of the spine.