The Kindle versions of Substrate Wars books are still cheap — $0.99 for Red Queen, $2.99 for Nemo’s World, and $2.99 for the latest, Shrivers.
A new option: the Substrate Wars Omnibus, all three books in a single e-book, now available at many non-Amazon book sellers and subscription services:
Apple iBooks/iTunes, $7.99
Barnes and Noble Nook, $7.95
Kobo Books, $7.95
Page Foundry, $7.95
Scribd, Unlimited subscription reading.
24Symbols: Unlimited subscription reading.
Tolino, apparently using iTunes outside the US.