Google Playbooks is too huge - 1.4GB
I can't tell you why it's using this much on your phone in particular. I just looked at Google Play Books on my phone, which is using 224 MB, of which only 32 MB is the app itself, and the rest is data (downloaded books).

Some PDFs and e-books can be very big, because they can include large pictures. Some e-books are high-quality scans of every page of a paper book, which can be much larger than 50 MB.