Are you on a Mac? If so, you can use Stanza desktop program to open the .pdb book and then transfer it to your i-device. In some cases, the Palm document, although .pdb, may be in a special format for a particular reader or program. You could also explore Calibre and some other programs that can translate or convert the docs you have.

Now, the other way, and it's faster, is to find the books within Stanza on your i-device via a wi-fi connection to Gutenberg or Memoware sites. Yes, you'll be redownloading them, but it's fast, it captures metadata (author, category, etc.) which makes it easier to browse later.

Another way may be to install eReader, another free i-device book reader that was very popular on the Palms... many .pdb books are in eReader format and if you can bring them over, eReader may be able to read them. IIRC, I brought over the eReader Merriam-Webster dictionary that way (that light version included with eReader Pro on the Palms).