if you haven't already you might want to look at this book:

http://www.amazon.co.uk/Penguin-Design-Cover-Story-1935-2005/dp/0713998393

it focuses on the covers but would be a good starting point

Off-topic slightly, but this might be interesting
http://www.penguincollectorssociety.org/pubs.htm#bydesigners

Tim

This is a good one to look at:

Jan Tschichold, Designer: The Penguin Years
by Richard B. Doubleday
Oak Knoll Press

I strongly second Jan Tschichold, Designer: The Penguin Years.

As a side note, I had this book on order through Amazon for months.

Save yourself the aggravation and buy directly from Oak Knoll. They are a good company.

Look for Penguins published 1948-9, in second-hand markets.
In Ruari McLean's "Jan Tschicold, typographer", there is a reproduction of the "Penguin Composition Rules", 1947, which was composed as if it were (4) pages from a Penguin, according to the rules. Nice bit of self-reference.

However, although he left Penguin in 49, the book interiors continued to follow his Rules for many years.

Anybody know when they stopped?

Yes, the McLean book is good, as well as, although the text is in German, "Leben und Werk des Typographen Jan Tschichold" by Werner Klemke, includes a bibliography, published by Verlag der Kunst in Dresden, 1988. With some info on the Penguin books, but also illustrations of many projects, type and design. Worth having, if you can find it.

I second Mclean's book: Mclean was Tschichold's under-study at Penguin - I'm not sure who left first? However, I would definitely not recommend 'The Penguin Years' by Richard B. Doubleday: the book has an incredible amount of inconsistencies and inaccuracies; furthermore, Phil Baines gave the book a scathing review in Eye Magazine:

http://www.eyemagazine.com/print/review.php?id=134&rid=677

I would try 'Jan Tschichold: A Life in Typography'; 'Jan Tschichold, Typographer'; 'True to Type by Ruari Mclean; and then email St Bride library in London to see what they have in the way of books and advice.

You can find a lot of design related for books on www.zinio.com and also www.myfavouritemagazine.co.uk. I am also a designer looking out for more design related stuff on the web.