Some Ars Magica supplements are published with soft covers, and others with hard covers. The reasons for the difference are related to Atlas Games's publication costs and the retail price of the books.
 Hard Cover supplements
Starting with Blood and Sand in 2002, Atlas Games began publishing supplements with hard covers.
In September we'll be releasing Niall Christie's Blood and Sand: The Tribunal of the Levant. As a $30 128 page hardback.
Why the price and format change? Basically, Ars Magica was only barely viable at the old rates. The hope is that the price rise won't affect sales too much, and that we will therefore make significantly more money on each publication. This should allow us to pay writers and artists a halfway decent amount, and should mean that ArM can justify its warehouse space. We're raising the price to make more money. We're going to hardback because it's not much more expensive than softback would have been, and you get a sturdier book. (Not all ArM books will be hardback -- Black Monks will probably be 64 pages, which would look silly hardbound.)
The ideal would be to attract lots of new gamers, but that's a job for ArM5, and the line has to be viable in the meantime.
--John Nephew, Berkeley Ars Magica Mailing List, June 6, 2002
 Soft Cover Supplements
Before 2002, all Ars Magica supplements were published with soft covers (as were the first four editions of the core book, with the exception of a small print run of hardcover copies of ArM4).
Softcovers are much more affordable to reprint, including print on demand, which is an option I want to keep open for a book of adventures. We haven't published any adventures in a long time for ArM, and I am not sure what the demand will be.
Future sourcebooks are still planned to be in the familiar hardcover format of past ArM5 sourcebooks.