Essential reading.

Mark Gatiss has a bit of a mixed record on Doctor Who. He wrote a few rather ill-regarded stories for the Revived Series, and a few that are generally pretty liked, but not exactly loved. But here, in 1992, he wrote what is in my opinion his greatest contribution to Doctor Who. It is for his work on this that I forgive a couple of his televised duds.

When Tom Baker took on the role of the Doctor, as wonderfully as he did it, he did have a certain "invincible" feeling. The Hinchcliffe/Holmes stories tended to keep him grounded by constantly putting him through the ringer, putting him in intense pain, burdening him, life-or-death situations, gods, monsters... And then in the Graham Williams era that seemed to stop. By the time JNT came on-board and was challenging him again, it felt weird that Tom's Doctor was capable of being vulnerable. (The melancholy vibe Tom had during season 18 certainly helps this; whether you believe this was intentional or not.)
Peter Davison then went on to have it pretty easy for most of his run, and Colin and Sylv had something of a deification going on. Mark Gatiss, it seems, isn't into that sort of approach, despite it being the fashionable tack in the VNA range; Mark wanted to really challenge the Doctor. Really get Sylv's Doctor vulnerable. Like the early Tom days, like Caves of Androzani, like the Troughton era, like Daleks' Master Plan...

And boy, did he succeed.

The most direct precursor to Doctor Who is Quatermass; a trilogy of 6-part science fiction serials broadcast in weekly half-hour parts in the 1950s. (The Quatermass Experiment in 1953, Quatermass II in 1955, and Quatermass And The Pit in 1958/59. There was also a 4-part serial for ITV in 1979, but it wasn't very good.)
Mark Gatiss is a long-time admirer of the creator of Quatermass, Nigel Kneale, and in fact as I write this, it was still pretty recent that Mark took to the stage to play the lead role of Professor Bernard Quatermass in the 70th anniversary rehearsed reading of the complete scripts for 1953's Experiment. (Particularly notable since, like a lot of early Doctor Who, there are no surviving recordings of the last four episodes of Experiment. The latter two serials, however, survive in full.)

Why am I telling you about Quatermass? Well, for one thing, this book is partly a tribute to Quatermass. One of our lead characters, Edmund Trevithick, is essentially a stand-in for if William Hartnell had played Professor Quatermass, and hadn't suffered quite so terribly from arteriosclerosis. He's a somewhat bitter old man, longing for the old days...

Nostalgia.

In a way, that is the villain of Nightshade. There's a plot here to do with an ancient evil (somewhat reminiscent of a Hinchcliffe/Holmes monster), and the monster preys on the wistful, bitter, and painful nostalgia of our principal players. Yes, even the Doctor. The Doctor begins this story in an unusual bout of melancholy, and in a sense, this whole book is about grief, melancholy, a longing for the past, and how this can strangle us, destroy us, and stop us from really living.

Ace gets a great showing in this story, standing on the other side of things to the Doctor. Her past has been traumatic, and from that, she's learned to look forward.

What does all this amount to? A bloody good book, that's what.

Timewyrm Revelation explored what Doctor Who could be with limitless budget and time... You could almost imagine a version of the story presented as a mid-budget TV movie in the '90s, shot on film, co-financed by an American distributor as a show to compete with the likes of Buffy the Vampire Slayer. This book, meanwhile, is more interested in running with what Doctor Who for adults looks like. Similar to Warhead, the big draw isn't necessarily the unlimited budget and time (although Warhead certainly would be nigh on impossible to shoot for TV, even today), but rather the draw is for a complex, layered, deep, emotional story. Mark knew this book would only really be read by adults, and older teenagers, so he wrote something for adults.

It's layered, it's sad, it's fun, it's melancholy, it's deep, it's human. It's beautiful.

This may be my favourite of the VNAs so far... It and Revelation stand together, at least, as the two heavy hitters.

Essential background: The Dalek Invasion of Earth (season 2)
Extensive background: Cat's Cradle: Warhead (VNA #6)

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 Extensive background for some additional details you may find interesting.