Honestly, I think you've got a few wires crossed, but I get where the question is coming from. There isn't really an official standalone book called a 'Devil Fruit Encyclopedia' that you can just find. The information is all within the pages of the manga 'One Piece'. You're probably remembering those detailed info pages Oda draws sometimes, or fan-made wikis that compile everything. Those fan wikis are your best free resource, honestly. They've got every fruit categorized by type, user, and arc, with breakdowns of abilities and weaknesses.
For reading the actual manga legally and for free, your options are a bit more limited but do exist. The Shonen Jump app and Manga Plus app have the latest chapters available for free, but their back catalogue is behind a subscription. You can sometimes find those specific encyclopedia-style pages by searching for 'One Piece SBS' or 'Vivre Card' info online, but again, that's fan-scanned content. If you're just after the raw data and not the story, I'd stick to the wikis. They're surprisingly comprehensive and updated by dedicated fans who track every tiny detail Oda reveals.