This is one of the classics of heraldry, and is full of mostly good information. Unfortunately, one of its major advantages - the number of engraved images - is a drawback here. The original was printed in black and white. Colours are an essential component of heraldry, and there has long been a tradition of representing the various heraldic colours with engraved lines - horizontal, vertical, cross-hatched, etc. This works perfectly well for an engraving, but not so well when those engravings are scanned. "Red" (vertical lines) and "blue" (horizontal)" come out ok, as does "black" (horizontal and vertical cross hatched) most of the time, but there is no difference between "silver" (blank) and "gold" (dots), or "green" (diagonal left to right) and "purple" (diagonal right to left). Increasing the size of the image shows that the lines simply haven't registered during the scanning process. This is in part due to the comparative lightness of the original printed ink. This is one place where I would suggest a rescan at much higher resolution.
That doesn't mean that the illustrations aren't useful, but at times it can be hard to make out distinctions between fields or charges, and for most it is impossible to determine the original colours intended. Fortunately the text does provide that information, so for those simply looking for images of heraldic charges, fields and divisions to work from, it's a little frustrating at times, but otherwise fine, and for those looking for specific heraldry, the information is provided.
As for the content of the book itself - it really depends on what you want it for. For someone who has the basics of heraldry down, but who is struggling with the details of blazon, this isn't bad. As a comprehensive guide to how heraldry works, I would recommend Fox Davies first - this is definitely a dictionary, not a teaching manual. Like Fox Davies, it's strength is also it's weakness - in trying to be comprehensive, it gets difficult to wade through and can be overwhelming. For someone trying to unravel blazon, or work out some obscurity from a heraldic text, this would be very useful, but trying to work the other way it is less helpful.
It is definitely a product of Victorian-era British heraldry - there is little to no recognition of other European schools of heraldry.