Avoid at all costs.

Let's not beat about the bush, this book is bad. It's very bad. The Doctor and Ace are characterised all wrong, there's some really stupid fanservice towards the end of the book that has no business being there and comes off as incredibly lame, it's boring for long stretches, it's hideously sexist, it handwaves some seriously horrible things (primarily sexual harassment and assault) done by the historic characters as "products of the time". It's an awful, terrible, very bad book, and a horrendous way to start the series. Do not read this. If you're really curious, read a plot summary. Anything else you need to know is more than adequately explained as it comes up in the following three books.

Not only is this book insulting, it's also boring. Do not bother.

Recommended background: None
Worthwhile background: The Invasion of Time (season 15), Ghost Light (season 26)

In my reviews, particularly of the Virgin New Adventures, I prefer to use a 4-tier system of grading:
Essential reading - If you want to read all the best VNAs, get all the most memorable story arc beats, and generally enjoy the VNAs without having to trudge through the mediocre/bad books, or perhaps even if you just want to pick up a good Doctor Who book with no intention of reading the entire series, look for this rating.
Worthwhile for extensive reading - Not outstanding, but I won't outright tell you to skip it if you want a sense of the VNAs overall. If you're determind to only read the best, skip these, but for a read-through of the series, I wouldn't skip them. They're the worthwhile, good-but-not-amazing books. You'll get a stronger sense of character arcs, story arc beats, and the growth of the VNAs as a range if you read these, but it will also take you a lot longer.
Not recommended - Not very good. If you really want to maximise your experience of the VNAs, you could read this, but it's definitely not advised.
Avoid at all costs - An irredeemable lump of human fecal matter. Do not waste your time with this insult to the franchise.

In addition, I list Recommended background that you may find necessary for understanding the story in full, as well as Worthwhile background for some additional details you may find interesting.