Trump has the burden of proof (under US law): He has to show that what they wrote was false, and that it damaged his reputation. In fact, since he’s a public figure, he needs to prove “actual malice” (that they knew it was false, or didn’t care whether it was true or false). Sounds like an easy defense if they did any due diligence at all (there’s also the doctrine that if details are wrong but the basic idea is right, they are still okay (in this case, if (for example) the book was a Christmas present and not a birthday present, the WSJ is fine). (All this is ignoring another defense - that of “opinion” because whatever else they did, the WSJ did make a statement of fact).