be of 后可接两种不同性质和类型的名词，他们的意义大相径庭。
1. be of 后可接value,  importance,  use,  help,  significance, interest, benefit 等一些抽象名词，这样的结构用来说明句子主语所具有的作用、重要性或意义。  在这类抽象名词前我们可用一些形容词，如: great, little, some, any, no, not much等来修饰抽象名词，用以说明程度。如:
They are of great help to learners of English.
他们对英语学习者有很大帮助。
The book will be of great value to students of history.
这本书对学历史的学生将很有用。
The meeting is of great importance.
这个会议很重要。
This medicine is of no use. 这种药无效。
This matter is of no significance.这件事无关紧要。

What are his books of interest? 他感兴趣的书是什么？

This subject is of great interest to me.这门学科对我来说是很有趣的。

It is a question of interest to biological science.这是生物学长期以来感兴趣的问题。

在“be of + 抽象名词”结构中， 其意义就相当于 “be + 与该抽象名词同词根的形容词”。如：be of value→be valuable,   be of help→be helpful,  be of use→be useful,  be of importance→be important,    be of significance→be significant等。它们之间可以相互转换， 因此，以上各例句可转换为：
They are very helpful to learners of English.
This book is very valuable to students of history.
This meeting is very important.
This medicine is useless.
This matter is insignificant.

2. be of +名词，还可用来说明句子主语在度量，大小，颜色，类别等方面的特征。这类名词有 colour, age, size, height, weight, shape, type，kind, quality等。在这类名词前常用different, all, the same， this, that,  a(n) + 形容词等词来修饰或说明。如：
Coins may be of different sizes, weights, shapes, and of different metals.  
货币可能在大小、重量、形状和铸造的金属方面都有所不同。
These pens are of many different colours.
这些笔有许多不同的颜色。
The two rooms are of a size / of the same size.这两个房间大小一样。
He is of the same weight as his brother.他和他弟弟一样重。
The productive forces were of a very low level.生产力处于非常低的水平。
City Lights and Modern Times were of this kind.“城市之光”和“摩登时代”就是这种影片。    
注意：此类句型中的某些表达可以转换成  “be + different / the same + in + 抽象名词” 的形式。如:  
Coins may be different in size, weight, shape, and metal.
These pens are different in colour.
The two rooms are the same in size.