I can't find clear details on the number of titles available (public domain vs commercial ebooks), but I do know that Play's partner boasts that they have 2,500 titles in Epub, Mobi, and PDF.
The Czytelnia service was developed in partnership with the Polish ebook startup Legimi, which offers its own similar service for 19.90 zloty per month. This name might be familiar to you; it briefly popped up in the English language tech news in late 2011 and then again in December 2012. It was tipped as being one of the hot prospects for the title of "Spotify for ebooks". Obviously that hasn't happened, which is itself a rather interesting story.
If I had to guess, I would bet that Legimi has yet to succeed because they were laboring under the same handicap as 24Symbols, Bookmate, and other non-US ebook startups. None of those startups are based in the US, the one country with the biggest ebook market and the 5 largest sources of ebooks.
Tell me, what are the 3 most successful ebook subscription services today? I would say Oyster, eReatah, and Scribd (assuming we exclude Kindle Owner's Lending Library).
It might sound odd, but at this point I am beginning to think those 3 startups all owed their success to where they launched (to some small degree). I can't help but wonder if the lack of language and cross-border legal complications helped smooth the way, and I also think that being located in the US lowered their travel costs when they went to negotiate with HarperCollins, Simon & Schuster, and other US publishers. These kind of deals can take a year or more, with numerous meetings. A flight across the US costs about a tenth as much as one across the Atlantic, and that adds up.
Then again i could be wrong. But if this trend is real then it could mean that the US will continue to be the center of innovative ebook enterprises for the near future. Thoughts?