The Reluctant Swordsman is definitely a fun read, though it's worth noting that it's the first of three books. It does tell a complete story, so if you decide to read it and stop, you're OK. I'm a big Dave Duncan fan. He's got a few stinkers (i.e. Hero, and the current faerie trilogy), but he's usually very good.
The Time of the Dark is also the first book in a trilogy, but it does not tell a complete story. If you get that, you'll need The Walls of Air and The Armies of Daylight to finish it. It's post-apocalyptic fantasy. That is, the world is ending, but the world is a fairly typical pseudo medieval Europe world, and the cause of the end is a plague of vicious nocturnal flying magical critters. It left an impression on me when I first read it at 18. The trilogy is worth reading, but it has a really disappointing cop-out finish.
Spellsinger is featherweight crap. Wannabe rock star gets transported to another world and discovers that he can make magic by singing. Other world is inhabited by cute anthropomorphic animals. The protagonist's mentor in the new world is a wizard turtle. It's essentially Foster trying to cash in on Xanth's popularity. The first book isn't so bad, but completely forgettable.
I've heard I Have No Mouth and I Must Scream is an absolute classic. I'm not personally interested because I really don't like Ellison or his writing.
The Forge of God is an excellent but very depressing end of the world scenario. Alien ships show up at various points on Earth, and one of them has a stowaway who warns the aliens are here to kill us. Things are not as they seem. It's a fairly complex novel, and it's more-or-less hard SF (that is, reasonably plausible and carefully thought out).
I haven't read Song of Kali. It's supposed to be good, but it's horror, and I'm not that much into horror. Simmons being a highly unpleasant nutter has put me off Simmons recently.
I don't remember much about Encounter with Tiber, other than it's about a mission to intercept a comet. I vaguely recall it being OK, but not exceptional.
I used to like Timothy Zahn, but stopped after the first Conquerer book. Blackcollar is very early Zahn, and quite readable, if not terribly deep. The premise is that humanity had a war with aliens and lost, and is now a subject race. Blackcollars were commandos with enhanced reflexes who used low-tech weaponry because the aliens were too good at detecting metals and power sources. This is a post-occupation story about a small cell of blackcollars.
These books are all pretty old, and generally available at bargain prices. My take: definitely get the base group if you haven't read either The Reluctant Swordsman or The Time of the Dark. Beat the average only if you haven't read The Forge of God and you think you'll like either the Ellison or Simmons book. The average is $10 or so, and I'd say either book in the $12 group is worth an extra $2 if you were already going to spend $10. Neither is worth $6, IMHO, so don't spend $12 just to get those two books.
The pricing schedule is odd, because the $12 books are some of the weaker books in the group. If I'd been structuring it, I would have used 2 books from the second tier as the $12 books, as long as neither one was Spellsinger.