Whoever the five of you are who nominated me for Citizen Recognition I am truly touched
This is a review of Troll Lord game's Mystical Companions. I got this as part of a humble bundle, so I am reviewing the 210 page pdf.
The pdf isn't hyperlinked. The text is black on yellow (so it will be ink intensive for printing). One thing I noticed was there was not a lot of art. There is one full page piece at the beginning of each chapter. So 13 pages out of 210 (+1 more for the cover).
The book is designed to introduce animal companions and familiars to 5e games. The primary way it does this is by new paths. Paths is a term created in this book as a catch all for the choices classes make at 3rd level in 5e. And here there are new choices for every class, granting them familiars or animal companions. With 5e being structured the way it is, this is an elegant (if intensive) way of doing this. 114 pages of intensive. With 12 pages for art, this is 100 pages of pure text.
Which brings me to my first comment: The Player's Handbook (D&D 5e) spends 68 pages on its classes and these are art filled. So to have 100 pages of pure text introduces rather a lot of options. In addition there is a lot of verbosity here, both in the presentation and in the sheer number of animals presented. The style is mostly easy to read but I think it is a serious question for a gamemaster - do you want to hugely expand the player options on offer?
My second comment is this: How the familiars and animal companions are structured very much reminds me of Pathfinder. I know Pathfinder imported 3.5 which imported 3.0 and these rules may have come from somewhere else but it strikes me how similar they are. A familiar has hit points equal to half of that of its master. It can attack using its master's proficiency bonus. It gets bonuses to its natural armour as the master levels up. It gets empathetic link and speak with master.
This feeling is compounded in that chapter one also focuses on feats. Although it calls them 'advantages'. Is this a Castles and Crusades thing? As advantage/disadvantage is a core 5e system it seems weird to introduce something called advantages. It's only a little weird. But they are alternative feats, focusing on familiars and animal companions. There are almost 30 pages of feats which gives it a very Pathfinder feel.
The only 'import' that I noticed that stood out as not quite imported smoothly is when it was talking about animal tricks. The tricks had a description and a DC which I instantly recognized as how this was meant to work from Pathfinder. But I don't think it is ever mentioned in the core rules of 5e, and and overview wasn't in the book. This was the only thing that stood out and it otherwise read like a 5e book. (But it felt like Pathfinder if that makes sense?) It also introduces a new condition - 'dazed'.
Each class gets several options to bring in a familiar or animal companion. Reading through they seem flavorful, and well balanced. It is this section that takes up most of the book.
Then we have the appendices, which are stats for creatures, a few new spells, and a rather cool concept of being a dragon rider.
For a prospective purchaser I would be asking this question before purchase - Do I think the absence of complex options and subsystems is a strength of 5th edition Dungeons & Dragons? It's a serious question and I don't think there is a right or wrong answer. I think 5e's strengths include its lack of complex options. I think 5e's limiting of familiars to a spell, and having them have 1HP is a very good way of handling things. I understand that they are less awesome than in previous editions, but they are simple. And the spell means a familiar can be dismissed when needed (or forgotten about).
That said if you want more options and subsystems and familiars and animal companions this is what the book is all about.