I do own this, so let me give you my breakdown of its applicability:
Chapter 1: History
Not very useful at all; just a brief recap of the 16th century, from 1500 to 1650, along with lists of the rulers of various European nations. It could possibly be of some use in fleshing out the Savage Baronies, depending on how closely you want their histories to match with their RW counterparts, and how well you can/choose to mesh them with the histories as given in Red Steel and other SC sources. Of course, given that certain nations are depictions of the same RW countries as they stood in different time periods (For instance, Texeiras and Vilaverde, IIRC, are both Portugal at two different times- though both, IIRC, would fit in this time period, so maybe still workable).
Chapter 2: Daily Life
This section would be more useful than the previous. Gives some general info on gender roles in 16/17th century Europe; homes (including some maps); fashion; currency; religion; science; equipment lists. Again, all of varying usefulness, and all of course geared to a low- to no-magic world.
Chapter 3: Characters
Getting to more technical stuff now. Character kits, primarily. I'll break them down further:
Fighter Kits: Four are presented- the Gentleman Adventurer (the most common one, representing soldiers/mercenaries); the Forester (basically rangers or gamekeepers); Clansmen (highlanders, non-city types); and Sea Dogs (which would be especially suitable for Vilaverde/Texeiras).
Rogue Kits: Vagabond (general thief types); Picaro (wandering rascals); Courtier (swashbuckling dandys).
Cleric Kits: Preacher (ministers of a church, missionaries, etc.); Protector of the Faith (a sort of paladin-like cleric or Knights Templar figure). The cleric kits are curious in that the Preachers can only cast spells they receive as bonuses for high Wisdom, and with many restrictions (spells are weekly, etc.), whereas there seems to be no limit on the Protector of the Faith's spellcasting. The restricted spellcasting fits in well with the RW setting, but it seems incongruous that one kit has full casting, while the other does not. I've often wondered if this was an error that crept in during editing or not.
Wizard Kits: Scholarly Mage. These are the only types of wizards (besides Necromancers, who are only allowed as NPCs). Their spellcasting is much slower (all casting times are increased one increment; rounds to turns, etc.)
In the High Fantasy world of Mystara, some of the above kits would be more useful than others. In particular, the spellcasting kits would need to be modified to make them useful mechanically. The fluff of them all would still be useful, though.
The rest of the chapter consists of things like social standing and an honor system that would both work in a SC campaign.
Chapter 4: Military Life
This could be very useful. Info on typical military structure of the time period, as well as armor and weapons (including gunpowder weapons). All in 2E format, obviously. There is also information on Fighting schools (which could be used in conjunction with the Secret Passes
rules (and upon which those were probably based).
Chapter 5: Four Wars
Fluff going more into detail on the major wars of the RW period in which the campaign book is set. Again, could be useful, might not be. It would require some degree of work to put into a Mystara-ized context.
Chapter 6: The New World
Brief overview of the new discoveries the Age of Exploration came across (Americas, Orient, Africa, etc.) Not really useful. Only one page.
Chapter 7: Folklore and Fantasy
Discussion of the fantastic tropes that have their basis in RW mythology, but which are standard in a High Fantasy setting like Mystara. Things that might be of use here:
[*]A listing of popular superstitions (for atmosphere or adventure hooks)
[*]Listing of some Monstrous Compendium monsters and where they might be found (ie, their RW origins)- could be used to place certain creatures in SC kingdoms more "organically".
Chapter 8: Sample Adventures
Not very useful. Three pages of overviews, one of which is tied to a historic RW location.
For further reading.