“故乐为之序”的意思应该是说“很高兴为某书/某文章写序言”,

This is a very simple and straight translation. It might not go so well if the rest of the article is stylized (as in an old-style Chinese essay). But, that still should be acceptable.

Just anoter way to put it.

Here we get into the tricky area of writing conventions, and how much you "domesticate". This is a rather formalised phrase, and this kind of phrase doesn't fit very well in this position in an English introduction. It would be more natural to place it at the beginning of the introduction:
"It is a great honour to be asked to write a few words for inclusion in this classic manual on beekeeping."

To keep it at the end, you'll need to do some work to make it link up with the rest of the text. To take the example from the link Dariush gave:

"It is only after spending many hours with Fred that you begin to realise the depth of thought and hard work which he pours into each seemingly effortless scroll; and that all his years of dedication have been so that the art of calligraphy can remain a vital tradition. This is a goal that we share, so it gives me great pleasure to write this introduction to a collection of his work."
與炳煌先生識之越深，越能體會他在各方面的努力，而這些努力只有一個目標，就是傳揚傳統的書畫藝術。而他也成功的將各種的體會融匯於個人的書畫藝術，這可是一般人所不及的。謹寫在張炳煌個展作品集付梓之際，樂為之序