I pre ordered a book that was to be available 5/14. I received notice that it was available but when I attempted to download "no longer available" or some such. Now I don't get the invitation any more. Please help.

Hello there, LowTexan.

I was able to locate a good article that gives a great overview of the pre-order process for iBooks.

iTunes Store: About Pre-orders

http://support.apple.com/kb/HT5714

Particularly the following:

Downloading a pre-ordered item

When a pre-order item becomes available, you will receive an email letting you know your pre-order is available for download.

If you've enabled iTunes to automatically download prepurchased content, then your pre-order content may already be downloaded to your library. If not, simply click the download link provided within your email notification, and your pre-order content will begin to download. Alternatively you can use the "Check for Available Downloads" feature in iTunes on your computer by choosing Store > Check for Available Downloadsto begin downloading your pre-order.

Thanks for coming to Apple Support Communities to find answers to your question.

Cheers,

Pedro D.