Many readers, and myself also, are too far off getting the first book done to even contemplate this particular writing fear but the fear that you only have the one book in you is actually a common writing fear and it can stop your progress before you have even completed that first book!
This in some ways is similar to the fear of success. It is fearing a situation that is not necessarily inevitable, you have to get that first book written for it to have any substance, but then if this is a strong imposing fear than that is perhaps exactly the reason that first book is not getting written. As long as the first book remains unwritten it is not a fear you will have to face and herein lies the problem.
As with many fears it is beneficial to localise the situation and address what it is you actually fear. It is highly likely your fear is as a result of a low-opinion of your work and you have perhaps surprised yourself with your first piece- great- but now you have to follow it up. It is helpful to remember that writing is a learning process and you are always growing so why would your future work be of any poorer quality?
A good comparison is to musicians. Many artists that have a great debut are under immense pressure to produce a follow up that is not just as good but better. Unfortunately it is a fact of life that this is just not always possible and it is not necessarily a measure of the artist (or writer), it can be a result of market changes, new competition or the artist has taken a different direction that is not as well-liked.
As I have said it is important to remember why you write in the first place and if you focus on the writing itself everything else should fall into place.
On a final note many writers, JD Salinger for example, have only ever had one book published and still been immensely successful.