This is an extremely well-done bit of Keeper prep that saves the busy Keeper quite a bit of time and effort. It contains 20 different tomes that are described in loving detail and include system agnostic information that is easily translatable into your Cthulhu gaming system of choice. You are required to do a bit of work to translate the books into SAN losses and/or Cthulhu Mythos knowledge (to use the Call of Cthulhu system as an example), but they provide a useful guideline phrase that gives you a sense of where those numbers ought to be.
The detail in the entries is fairly deep -- with thorough descriptions of the physical characteristics of the book, a detailed discussion of how the book was found or created in the first place (and, btw, they aren't all "books" -- some are carven stones, etc.) and where it might be found "today" (whenever "today" is in your game). Each entry is chock-full of the kinds of details that make it easy for the player to visualize the book, and the kind of information that the Keeper can dribble out to them as they research the item. In several cases, there are differing editions of the work as well, meaning you actually get quite a few more options than you might think for providing the information to the players. I can see this as being particularly useful to a Bookhounds of London (Trail of Cthulhu) sort of campaign. In short, very well done. One note of warning though; as a general rule, the books in here are more written for gaming eras from the Gaslight era (1890's) on; though with a bit of work (quite a bit in some cases) you could probably convert them to the dark ages or Roman empire eras. Still, in my limited experience, those earlier eras aren't quite as dependent on scholarly research (as opposed to information gathering by talking to people or physically surveying a given site) as the later eras are, so your mileage may vary, and since the original sources of the books are usually described (the soapstone carvings that Darwin recorded, for instance), you can possibly have your earlier era characters stumble on the originals anyway.
Do you NEED this book? Not at all -- a good, imaginative Keeper or GM can easily develop books just as complex and rich as these; however if you're as busy as most of us are, you will definitely WANT this book; it will save you hours and hours of work that you can instead devote to other areas.
Physically, the quality of the PDF is excellent, with no scanning issues, good margins, and no coloring problems to make it hard to read or anything. I didn't notice a single misspellling or grammar error in the book either -- which, in today's world, is frankly rather amazing. All in all, well worth acquiring for your Cthulhu gaming needs. The price is good at $3.00 on sale, or $4.00 regularly, so all-in-all, I'd say this is well worth the purchase price.