I don't have full details on it yet, but it looks to only be available on Claro's Android phones and tablets, but not iPhone or PC. Billing is done via the monthly phone bill, so there's a reason for that restriction. The service costs R$4 a week (about $2USD), and users can borrow up to 3 titles each week.
It's not clear how many titles are included, but I have to wonder if they have enough titles to make it worth the expense. There are currently 11 thousand ebooks available in Brazil (out of some 50 thousand titles published yearly), and I don't know how many of those titles can be had via Claro Leitura.
This makes the third ebook subscription service to launch in recent memory. The first, and the one to get the most attention, was 24Symbols. Then back in March Skoobe launched a similar service in Germany.
If anyone has more info on this service, let me know. There seems to be a dirth of information on the web.