Locate the infinitive and determine how it is used.
a good book to read is great expectations.
infinitive:
use:

A good book to read is Great Expectations.

“to read” is an infinitive verb in the above sentence. It is used as an adjectival infinitive describing a noun book.

Explanation:

Infinitive verb is basically a verb with 'to' in front of it. It is a type of verbal (a word form from a verb but functioning differently).

There are three main types of infinitives

1)Adjectival Infinitive: It always follows the noun it is describing.

Examples: A good book to read. A nice way to say something.

2)Adverbial Infinitive: It describes a verb and needs not be near the verb it is describing  

Examples: To complete (adverbial infinitive) the task, he asked (verb) for my help. He worked (verb) day and night to make (adverbial infinitive) a living.

3)Nominal infinitive: It may act as  

a)Subject: To be or not to be is the question.

b)Direct Object: He requested to leave.

c)Retained Object: He tried to help her.

Answer:

Infinitive : “to read”

Use: Adjective

Explanation:

'Infinitive' is the basic form of a verb. Any verb that is prefixed by 'to' is an infinitive. For example: to love, to read, to run and so on.

In the given sentence, the infinitive is 'to read' and its used as 'adjective' here. Infinitive can be used as Noun, verb or adjective. To find out how an infinitive is used, break the sentence into 'simple subject' and 'simple predicate', locate the infinitive with 'to'.

If infinitive modifies the noun, it is used as adjective. In the given example, to read is modifying the noun 'book', i.e. a good book ''to read''. So, 'to read' is used as an "adjective" here.

last one is the answer