Award-winning iBook Paperless by David Sparks is on sale for Earth Day:

I've never really properly celebrated Paperless winning one of Apple's Best of 2012 awards last December. Earth Day seems like the right time to do so. Through this Saturday, Paperless is $6.99. If you know someone that that could use some Paperless help, send them along.

I'm pretty close to telling anyone who asks me for iPad help that this book is required reading before we can talk.

I don't think they're on sale, but you should also check out Markdown and 60 Mountain Lion Tips for some more great Mac nerdery on the iBookstore.