The HR series was without a doubt the peak of expression of the AD&D Second Edition design philosophy. It gives an excellent high-level overview of Viking-Age Scandinavian culture and beliefs. It tells you how to modify the core rules to take AD&D away from its medieval base. You get which classes are allowed to native Norsemen, how those classes are modified, and new classes; a new system of rune magic for warriors; new price lists for a much more limited—and historically correct—equipment list; and all-new monsters, plus a list of which monsters from the MONSTROUS COMPENDIUM or MONSTROUS MANUAL are appropriate for the setting.
The book suffers from feeling like it was rushed. This is especially true whenever it mentions things you'll find in the back of the book—there is, for instance, no summary for players there as promised in the text. The author also seems confused with Second Edition terminology; for instance, the terms "wizard," "mage," and "sorcerer" are all used incorrectly when referring to classes. (Usually, where he says "wizard," read "mage"; where he says "specialist mage," read "specialist wizard," and where he says "sorcerer" read "wizard.")
Also be warned that the PDF you get here is the first printing. The second printing had errata corrected; it would have been nice to get that printing instead. The biggest piece of errata is on page 23; the paragraph is supposed to read:
"Although they possess magical abilities, runecasters are members of the warrior group. They can use any weapon and wear any type of armor. They fight and save like warriors. Experience is earned as a fighter and they advance in levels as a ranger or paladin. They are allowed to use all those magical items normally available to warriors. However, the runecaster never gains any bonus hit points normally due for high Constitution scores."
The scan of this PDF is adequate, not great. If they do a better scan of the second printing, my rating will go up to five stars.