Question: I am seeing that Google play books consumes 1.4 GB. I use Play Books only to view my PDF books and at most use 50MB of content. But I don't understand how it can consume so much space... Even a graphic game takes only 800MB. So can anyone point me out to why this happens??

Answer: 
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.
Google Play Books can also store cache and your data (includes your books).
Try clearing the application's cache by going to the application's app info.