There's nothing really written for the layman online (which I know of). You can start by googling for topics like 'latent inhibition' and research on meditation.

Genuine research papers-wise, you could join http://groups.google.com/group/brain-training to which group's files I have uploaded ~20 papers on various topics related to working memory and focus.

Important ones:

• klingberg2004-workingmemory-increase-helps-adhd.pdf
• mcvay2009-workingmemory-improves-focus.pdf
• thorell2008-workingmemory-improves-attention.pdf
• unsworthengle2008-wm-executive-focus.pdf
• jaeggi2008-nback-increases-iq.pdf

EDIT: in the future, Google will be deleting Groups' Files. You will want to search around for where the collection has moved to; I keep local copies of the most important ones in my wiki, and I copied all files c. October 2010 to my Dropbox account.