Go to https://www.packtpub.com/packt/offers/free-learning ,create account if you don't already have one, and click on the Claim your free ebook button.
They seem to be in Europe, so midnight their time is when the next days ebook is released.
You will have to go back every day to see what the next ebook is, the last book is now updated till 1 Feb.
To read the Raspberry Pi books, login then go to https://www.packtpub.com/packtlib/book/Hardware-and-Creative/978... I hope that link works.
I did post this a short while back, since then they have increased the free books end date and introduced the read online Raspberry Pi books.