I counted up the books I've read by him, and it's 18. The only other authors prolific enough to match up, I think, are Piers Anthony and Stephen King, so I counted them, too. King, I've read 13 (or 14; it's hard to tell sometimes, because his stories run together in my mind.) Anthony it's at least 31; I can't remember if I've read some of his, too, like the 2nd half of The Apprentice Adept series. Which puts Foster pretty firmly in second place, not a bad spot to be in my opinion.
The Spellsinger books, like a lot of the mid-80s fantasy I like so much, rely on a tried-and-true formula that works because Foster is a good writer and can rise above the cliches. The Spellsinger series follows Jon-Tom, a law student/musician from our world who gets accidentally pulled into another world by the wizard Clothahump, a turtle; in Clothahump's world, all animals -- spiders, insects, mammals, and birds, but not lizards (other than dragons) -- act more or less like humans. Jon-Tom's been pulled there because Clothahump thought he was an engineer; engineering in our world is like magic in Clothahump's world, we're told, although there's not that much evidence to support it.
The books themselves aren't short on action or adventure or imagination, from double-rivers (one underground flowing under the above-ground one to communist dragons to a mystical horse that must circle the universe in order to end it, Foster lays out a good adventure that moves briskly from one scene to the next, and the books manage to be a good, pulp-movie-style adventure.
The kid-who-has-powers thing was obviously big in the 1980s, and maybe Luke Skywalker started it or maybe he was just the latest iteration of it, but Foster's take on Jon-Tom's stumbling version of magic is kind of fun: he's a "spellsinger," so when he plays music and sings he can do magic, magic that's loosely based on the song he's singing -- so, for example, when he tries to conjure a boat in one book he sings Sloop John B. The magic has side effects or is uncontrolled, and in that Sloop John B example, Jon-Tom accidentally makes himself the first mate, and thus spends part of the voyage drunk because the first mate, he got drunk.
Jon-Tom's magic comes in mostly when Foster needs it to; although the books are about being a spellsinger, like most books in which characters have magic there are limits on the magic, both artificial and in-story. In The Hour Of The Gate Jon-Tom decides not to use his magic sometimes because he's too angry, or at others because the group deems it too dangerous, and so on. Whenever I read characters who have magic that have limits on it (for whatever reason) I go back to my list of dream stories I would like to write, which include:
1. A murder mystery in which the detective is himself the one who committed the murder, but he doesn't know it and the reason he doesn't know it has nothing to do with accident, drugs, or being drunk, or amnesia or the like -- in other words, he murdered someone, then has to solve the mystery, and the reason he can't remember it isn't explainable through hokey devices, and
2. A fantasy story in which there is an all-powerful wizard, one who can do pretty much anything he wants, and then... I haven't worked out the rest of the plot but there'll be no limits on his power.
The limitation-on-power thing bugs me, especially when the limit is arbitrary or unexplained. In All The Birds In The Sky the witch's power was limited by the need to not be aggrandizing, although that was never really explained. In the Xanth books characters are always trying to forego their powers and figure things out some other way, which strikes me as incredibly odd, and counterintuitive. Imagine this:
Batman and Superman are standing atop a skyscraper and a giant robot begins tearing down Gotham City. Batman says You should just pick it up and throw it into the sun. Superman says nah I think I'll try to outsmart it so it leaves the city alone.
What would Batman do? I think probably slap Superman and say don't be stupid, just pick it up and throw it into the sun.
(There are a certain amount of people right now saying well what if the robot is actually filled with some sort of explosives that, if launched into the sun, will destroy the universe? but that's not part of the setup, and you're sort of part of the problem.)
The problem is, I think, artificial conflict. And it works worst when it's used only whenever you want it to be a limit. I was watching The Force Awakens the other night when I couldn't sleep, and I was thinking about how Rey manages to mind-control the stormtrooper, and how Obi Wan did that in the first movie. Mind control seems to me to be exactly the kind of thing Jedi shouldn't do; if you can't get mad and attack someone without going to the dark side, how can you make someone else do something against his or her will? And why not use it all the time? I know, I know: it only works on weak minds or something, but the point is: there is a limit to Jedi power that is applied only when the story requires it. Luke can't get mad and fight Vader without going to the Dark Side. Rey can use her mind-control powers to make stormtroopers give her weapons so she can kill people. It's inconsistent.
There's a bit of limitation, for no real reason, in The Hour Of The Gate, even beyond Jon-Tom's occasional reluctance or lack of ability. Clothahump sometimes is too tired to do more magic, a limit that also gets applied somewhat randomly, and there are things magic can't do that, again, don't seem too consistent.
The point of it is that if your story requires a basic element to fail or not be used at a certain point to create drama, then the story itself isn't probably so hot, or the system you've set up (regardless of whether it's meant to be a system or not) isn't consistent and needs some work. All systems need consistency or people get removed from the story.
As an example, consider my objections to shows like Law & Order: I don't like to watch them because frequently they cut corners or do something that wouldn't happen, like two lawyers arguing a point of constitutional law as they walk down a hallway with a judge, who then rules, all off-the-record and with no witnesses or anything. That's ludicrous! I usually say.
Then, one time, watching the show Ed, about a lawyer who bought a bowling alley and works out of it, Ed The Lawyer got hired on some huge case that went to trial like literally the next day, with no witnesses or subpoenas or juries or depositions or anything. How come you don't get bugged by stuff like that? Sweetie asked me, given how unrealistic it was. The difference, though, was that Ed didn't pretend to be a realistic version of the legal system; it was a show about a bowling-alley lawyer. Law & Order wants to be deemed realistic, and so when it cuts corners for the sake of storytelling (it's more dramatic to have lawyers walking down a hall than simply sitting at a table, I get it) it breaks the system down.
If your limits are built into the story, like Superman's kryptonite, Jon-Tom's unfamiliarity with how magic works, then they're fine. It's perfectly fine to have Jon-Tom's magic fail at a key moment because he's been doing magic for like 2 days. If your limit needs to be created at that moment simply to create drama -- the wizard is suddenly too tired or magic 'doesn't work' on that thing or some such - -it's just poor storytelling.
That's not a critique of The Hour Of The Gate. Foster doesn't fall prey to the 'we can't do that so it's dramatic' bug all that often. This is a good book, as good as I remembered it being, and if you like pulpy-fun fantasy fiction Alan Dean Foster is your guy.