While FlipSnack has a paid version, there is also a free version with limited access. It isn't something you could use for a long time, but it could be used for short-term.
It's easy too. Just upload PDFs or JPGs to the site and click publish. With the free version, you have 1GB of storage, three 15 page books that can be opened 10,000 times a day. The booklet can be embedded but it does have a watermark, but who cares, it's free.
I think if someone is doing a unit and has a small collection of materials, this would be a great way to present it. There is also a paid version that allows for more things to be uploaded to your flipbook.
What an interesting tool, right? It is definitely worth checking out.