General question about eBooks

I have no experience whatsoever of eBooks so, please forgive me asking what must be a very basic question.
I have an Acer Iconia B1-850 tablet and rather than take hard-copy books on an upcoming long-haul flight I wondered if I could use the tablet to download an eBook or two before I go.
My tablet has Google Play installed and I am fairly confident about using that to purchase a book but what concerns me is will I be able to read it with the tablet in airplane mode.
Once the eBook has been purchased and downloaded does it reside on the tablet so that I can access it offline at any time?
Also, I appear to have 1.15 Gb spare storage capacity on my tablet. Is this enough to download say two novels?
Thanks.