Try to discern the reason for the Recluse's isolation, but don't force him to be sociable. If he's shy, find ways to interact without making him uncomfortable.
TREAD LIGHTLY
Allow the Recluse to be solitary, as long as it doesn't interfere with departmental work. If you thrust him into the office social center, he will only barricade himself in his office in the future. If he's your subordinate, your primary interest is in the quality of his work; if he's your boss or colleague, your primary goal is to continue to have access to him and to any information you need from him.
You might try routinely communicating with the Recluse via e-mail, the most unsociable of all social exchanges. Keep your communication all business. If the Recluse is your subordinate, you can set up a schedule for delivering any reports or updates, paper or electronic, that may be needed to keep you and his co-workers current on his progress on joint projects.
Let the Recluse know when information he supplied accelerated the completion of a project or significantly affected the outcome in a positive way. Be succint; there's no need to make it a social conversation. Give the good marks to the work, not the person. Be prepared to give a brief report on the same subject if you catch the Recluse out in the open, for instance, on the way to or from the restroom or elevator.
BARGAIN FOR PRIVACY
If the Recluse is highly focused and extremely valuable, the same kind of written reporting will work. But you can also negotiate periods of guaranteed isolation in exchange for scheduled meetings that the Recluse can prepare for.
Discuss the idea of setting up a brief but regular meeting for the Recluse and your team. Have questions and information requests submitted in advance, if possible. In return, you will promise to secure his work time and consolidate information requests.
If you manage a department or a work area, limit others' access to Recluse. If he is bombarded with requests for information, nobody will get what they want. If you coordinate the flow, everybody will benefit. If the Recluse is your boss, don't bother him with details and minutiae. Find out what is most important to him and use his need for that kind of information to gain access.
INVEST IN TRAINING
Like the Minutiae Monster, the Recluse may be the way he is in part because of a lack of skills. If this is the case, you can get the Recluse involved in peer mentoring or hire an outside trainer. Concentrate on building communication skills and educating him about interpersonal dynamics.
Make sure the training is relevant to your needs. Many organizations send people to training where business issues of all kinds are painted with a broad brush. Your Recluse may have a superior intellect and be a valued member of your team, so don't insult him with inappropriate or irrelevant training. Talk to the Recluse before sending him for training and explain what you expect and why.