"Are they difficult to come by?"
Not sure - The Horn Book was fairly easy to come by 20 years ago - not seen it for a long time though.
Rationale came in 2 large volumes (hardbacks) - the first ('clean' dirty jokes) was published in paperback, but I never saw a copy of the second ('dirty' dirty jokes) in Britain in any form apart from the one we got.
The Limerick and The New Limerick was published in paperback and was endlessly plundered in smutty paperback form, incomplete and minus annotation, of course.
"There were several books...."
Two other Legman works to search out are a pamphlet entitled 'The Fake Revolt' (Breaking Point NY 1969) and 'Love and Death' A study in censorship (Hacker Arts Books NY 1963) - both well worth having.
Jim Carroll