If you have any interest in the modern or ancient Maya, in Maya archaeology or the Maya inscriptions, you should buy his book! This is the only substantial dictionary of the modern Yucatec Maya language. All the other published dictionaries are either very limited in scope or contain mostly "Classical" (16th century) Yucatec. The authors' scholarship is unparalleled. This is an *authoritative* work and will always remain so. The very complete section on grammar is invaluable, since there is almost nothing similar; works like Tozzer's "May Grammar" are outdated, incorrect and incomplete. Andrew Hofling's recent Itza Maya dictionary is an excellent work too, but is organized differently and is, of course, about a different language. I use this reference book and recommend it very highly to others.
