I tried bird.makeup; for @google it succeeded, but for my own account it gave me a 500 error and suggested I enable development mode in order to get a more detailed explanation (i.e. the stock HTTP 500 response from the application stack).
If you're having any problems downloading the books, We reported the issue last night. It's just after 8:30AM right now Pacific Time so they should open within an hour.
Humble wrote us, "If you're responding to your users, please let them know we'll address as soon as we return to the virtual office in the morning."
Thanks for supporting our authors and a great charity, helping to de-stigmatize Mental Illness among young adults.