Second, it depends on context.
Right now the FBI has only authorized certain organizations to use the seal; if you're a member of any of them you should contact the organization to discuss it (the list in the link below). The lack of permission extends to websites. It is illegal to use the FBI Seal without permission.
The FBI does, however, have a standard written warning that you can use (without the seal):
“Warning: The unauthorized reproduction or distribution of this copyrighted work is illegal. Criminal copyright infringement, including infringement without monetary gain, is investigated by the FBI and is punishable by up to five years in federal prison and a fine of $250,000.”
As for putting a picture of it in your book, it would depend on how you do it. I suggest showing an attorney how you intend to use it in context.
You should carefully vet the people submitting materials for your book to make sure that they're not giving advice that they have no business giving (such as a non-lawyer trying to give legal advice).
For more information, the FBI's website may be useful. I've linked their webpage on the warning below.
For a consultation, e-mail firstname.lastname@example.org to set up an appointment. This answer is provided based on limited facts and does not create an attorney-client relationship, nor should it be relied on as legal advice. It is only a generalized, cursory examination and is not a substitute for retaining a licensed attorney.
No. The FBI has a licensing program pursuant to which it may decide to license u se of this seal. However, you are forbidden from using it without permission. The FBI notes, however, that until you are granted a license by the FBI, it is perfectly fine for anyone, without FBI approval, to use the following generic language on material protected under U.S. copyright law: “Warning: The unauthorized reproduction or distribution of this copyrighted work is illegal. Criminal copyright infringement, including infringement without monetary gain, is investigated by the FBI and is punishable by up to five years in federal prison and a fine of $250,000.” You cannot use the seal in your book as a reference picture until you are granted permission to do so.