I think this is because its target audience is clearly libraries (the publisher is Facts on File), and its intended readers are high school and college students who need to write a book report but don't necessarily want to read the book in question. It falls into the same territory as Michael Drout's TOLKIEN ENCYCLOPEDIA or Wayne & Christina's COMPANION & GUIDE in its attempt to cover Tolkien's life and (especially) works through a series of encyclopedia-like entries, but unlike those seems to be largely a one-man show. Rudd has clearly put an enormous amount of work in on this book, but I'm not sure how many Tolkien scholars will find themselves making much use of it: its intended reader is the non-specialist.
I haven't had a chance to read the book yet, but on a quick skim it seems that Rudd derives most of his information from relatively few sources (e.g., Carpenter and Garth seem to provide pretty much all the biographical information). I haven't found any signs yet of original research, such as distinguish the Hammond-Skull, while it lacks the Drout project's all-star cast, which largely offset that production's flaws.
As usual when first looking at a great big book like this, I check the relative accuracy and quality by reading a few entries on topics I know really well -- like THE HOBBIT. Here Rudd starts by saying that "The origin of The Hobbit is well-known" and then proceeds to get several of the salient facts wrong. Oddly enough, Rudd devotes three paragraphs to describing the existence and significance of the 1960 Hobbit but never mentions that it's been published. I suspect from this that his book was a long time in the works, and so wasn't able to take advantage of books published within the last half-decade or so (just as my own MR. BAGGINS wasn't able to draw much from the Hammond-Scull).
Rudd's method is to first provide a brief overview, then a synopsis of the text, then commentary on the book, then some notes on major characters, ending with a selective bibliography. In the case of THE HOBBIT, the whole entry runs some thirty-five pages (p. 95 to 129), of which fourteen are a plot-summary. The commentary that follows is a bit eccentric -- for example, he asserts that Bilbo is the Grail Knight and Bard the Fisher-King -- and its general thrust is suggested by his including in this entry's Suggested Reading: Northrop Frye, Joseph Campbell, Jung, Propp.
The sheer bulk of the synopses indicate that this really is a book meant for the non-specialist (cf. the paragraph or two summarizing each poem in ATB). But that being the case, if this book is going to be the only authority someone checks when reading about Tolkien, it's all the more important that it get the facts right. A quick skim makes me worry on that front (e.g., "Thorin is the grandson of Thrain, the last dwarvish King under the Mountain"), but I may have just been unlucky and come across more than my fair share of the sort of inevitable slips that occur in any large project.*** I'll certainly be dipping into it and reading more over the next few weeks.
So, my initial impression: an impressive achievement, but to be used with some caution.
current reading: THE ZERO STONE by Andre Norton 
current audiobooks: The Learning Company lectures on Skspr***
*i.e., 'you seem to buy everything there is to buy w. Tolkien's name on it; why not buy this too?'
**making it the latest of a recent run of budget-busting bks on Tolkien.
***Captions are a particular problem, as when a photo of the Radcliffe Camera states that "The manuscript of Roverandom was discovered among Tolkien's papers in this library" (p. 333). Um, no. Also, at least four times cover art for movie tie-in editions of various volumes of THE LORD OF THE RINGS are inexplicably described as "coloring book editions". However, these gaffs shd not be held against Rudd himself, since captions for most publishers are not written by a given book's author and are invariably inserted into books at a v. late stage, usually too late for even the editor to double-check them.
****not surprisingly, they think Shakespeare wrote Shakespeare. I gather the new movie just out about Skspr ("Anonymous") subscribes to one of the various conspiracy-theories instead.
UPDATE Oct. 17th: I've corrected the spelling of "Roverandum" to "Roverandom", as per D.B.'s comment -- JDR