I won't always be able to find free books, but when I find animal-based books -- or a book that just sounds interesting -- I'll post them at 8:00 p.m. every evening. I'll be posting them as soon as I find them during the day, but I'll have them re-tweeting at 8:00 so there will be a stable time that everyone can look.
I THINK the Hootsuite account is set for central time, but I'm not going to swear to it. If it is, the tweets with free books will come down:
9:00 p.m. Eastern, 8:00 p.m. Central, 7:00 p.m. Mountain, 6:00 p.m. Pacific.
Sometimes it will be Kindle in Amazon, and sometimes it will be B&N, so you will need accounts in both of those sites.