The book is, apart from the unorthodox binding and layout, entirely valid as a magecraft primer. What spells Laoise recognizes seem not terribly different from those she's familiar with, and the discrepancies can be attributed to mere differences in technique or the author's idiosyncrasies. The more theoretical chapters are trickier to examine, but as far as her knowledge stretches they range from 'far-fetched, but possible' to 'intriguing and probably true.' In short, this is a valuable book, but as far as the contents go it's not that far off from any magus' research notes.
However, no pocket journal full of scribblings on magecraft, even one as painstakingly thorough as this, would let an untrained boy use magecraft. After her examination, Laoise can only settle on one satisfactory explanation: The book isn't acting as an instruction manual, it's acting as a conduit. Someone, presumably the author, imbued this book with a link to their own Magic Circuits. Essentially, whenever Ren tries to perform one of the spells contained therein, he's drawing on the power and stability of a more experienced magus. It's not perfect - even with that wellspring, Ren wouldn't be any more powerful than someone with a few years of training - but with someone conditioned to draw on dangerous amounts of prana, simple rituals that were performed imperfectly might still go off with only minor flaws.
A link like that...It's certainly possible, but Laoise can think of quite a few reasons tools like this wouldn't see widespread use. One, naturally, is that it would take an expert to construct. Another is that it's limited in application. Two people's Magic Circuit configurations and numbers will be quite different. Connecting them requires intimate knowledge of both systems, and the connector is useless outside of those two. In this case, that means only Ren could use the book. Anyone else would just be drawing on their own skills as a magus. But most importantly, once a 'request' crosses a link like this, there's no way of denying it. Normally, a magus whose spell is going dangerously wrong can take countermeasures, but here the one performing the spell and the one suffering the consequences are two different people. This book is a blank check for some other magus' prana, and putting too much strain on it with a severely botched spell might kill its author.
...All the same, while that explanation is hard to believe, it's still within the realm of the possible. Unfortunately, if that is the case, it's certainly concealed well. Even when holding the book, it's hard for Laoise to tell the lesser magic used to change the pages is there. The link, if it exists, is most likely even harder to find and trace.
Extended magibabble dump, check. One step closer to stealing Nasu's job.
Another way to think of this is as a version of Sponsored Magic where the debt is being dumped on someone else, and accruing too much doesn't get you sent to pick up Mab's laundry, it probably just kills you outright in gruesome fashion.