Question: I want to write a series but I'm doubtful if my first book is a stand alone one. It has its own plot but there are events on it that will pursue and need to justify in the next books unlike what has been said that first book should be stand alone. Isn't tough to get this kind of book published nowadays?
To be fair to your reader, the first book should have a story problem that is resolved by the end. However, the resolution of that goal could set the stage for the next book in the series, perhaps by opening up some larger issues or giving the characters a new purpose which they will pursue in the next book.
It's a bit like writing a pilot for a television series. You want to introduce some continuing characters, the story world, and perhaps some overall series problem or mystery that will make the reader want to come back for more. But at the same time that first episode ought to be a satisfying experience, which requires some kind of resolution. Leaving everything hanging can be a frustrating experience for the reader.
This was the problem some people had with The Lord of the Rings
. They read the first book, not realizing it was the start of a trilogy, and felt annoyed that the story was left dangling.
At the other end of the scale, there are many series in which each novel is completely separate. The only thing they have in common is the main character and perhaps the type of plot (for example, Agatha Christie's Poirot or Ian Fleming's James Bond novels).
Established writers with a history of strong sales may get offered a contract to write a series. In such cases, they can get away with writing the type of trilogy where only the last book has a resolution, so the reader has to buy all three books to get a complete story.
I personally feel such cases are unfair to the reader who isn't already a fan of the author and buys the first book just to check him/her out, only to find out he has to buy two more books. It may have some benefit in terms of marketing, but at the cost of some integrity. It may also make some readers decline to buy that first book, because it requires a bigger commitment than they want to make with an author they haven't read before.