Almost every post will have some form of affiliate links within them for ingredients, supplies, or other reference materials associated with the post. (tattooedmartha.com)
Would Also to add I decided to bring a couple of other reference books to use while i was on my way to the meeting. (goodereader.com)
If you and your students want to pay for some good apps, there are many other reference tools to suggest. (edutopia.org)