The Elder Scrolls Online team has recently announced that from now to July 6th, if you buy The Hero's Guides to The Elder Scrolls Online from the Bethesda store, you'll get one free month of gameplay of Elder Scrolls Online with it. The Hero's Guides is a set containing three lorebooks and a small portfolio that has Alliance War art and the copy's number (from the limited run of 10,000) in it. I recently purchased it and read them, so I'm reviewing them (spoiler-free) to give you an idea of whether you think it's worth the steep price.
Agents and Reagents: The Bounty of Mundus The journal of Gargrell Sorick, an apprentice at the Wayrest Apothecary. Apart from the introduction and last few pages, the book follows the same format: illustrations of the different alchemy ingredients, listing their effects as seen in-game and some extra background info on them (such as how to harvest them). I rate this as my second-favorite of the three. Mostly this book will appeal to those really interested in lore and those who might want to use a little background info for fanfiction or RPing.
Gathering Force: Arms and Armor of Tamriel – A book describing the armor and weapons used by each race, written by Longinus Attius, an Imperial Fighters Guild hall steward. Like the first book, it follows a set pattern: a color picture of a champion from each race, and a couple pages with sketches of the race's armor and text describing what materials they use and how they're designed. Also like the first book, this will only appeal to those who are super-interested in these little details of the world. This was my least favorite of the three books. Though it has more named characters than the first one, they have no depth whatsoever and in most cases aren't even mentioned apart from their picture. At least in the alchemist book we get an idea of the personalities of the three major characters.
Kyne's Challenge: A Hunter's Companion The tale of a group of mostly Nord hunters and their adventure across Tamriel, as they attempt to fight as many different beasts as they can in order to pass along the information to more inexperienced Fighters Guild members. This is easily the best of the three books. It's an actual narrative rather than a collection of information, it is longer, has deeper and more engaging characters, and has some interesting plot twists along the way. It's definitely an enjoyable read.
Overall, the quality of the materials of the books is pretty good: they have soft faux-leathery covers, gilded or deckled pages, ribbon bookmarks, and so forth, with very nice illustrations. However, all in all, the content of the three books is nothing spectacular. The Kyne book was a good read, far better than the others, but the other two books were something of a disappointment, and the Kyne book alone does not make up for those. It was interesting, however, how some of the things in these guides tied into each other and also into the Improved Emperor's Guide to Tamriel that came with the Imperial Edition of ESO (for example, Flaccus Terentius, the author of the Emperor's Guide, makes an appearance in the Kyne book). In my opinion they did fit the Elder Scrolls feel, more so than the two Greg Keyes novels, I thought, and they did provide some extra background information. However, if you're just looking for a fun story, I'd point you toward the Greg Keyes novels (The Infernal City and Lord of Souls) instead.
Final verdict: If you're really into the lore and background information, and can afford it, it might be worth a look, and now would be a good time to do it, with the 1-month-of-ESO deal. I like having them books and Elder Scrolls are two of my favorite things but I would've been far more satisfied if it would have cost a more reasonable amount. Despite the good quality of the items, the content in them is not worth the $100 price tag.
If you're interested, head on over to the wiki – I've recently finished the pages for the books, and they contain detailed synopses in case you wish to know exactly what happens in each one.