One of the best sources for the Overland Campaign, in my opinion, is Gordon Rhea.
In the June 2004 edition of
North and South Magazine, Vol. 7, No. 4, Rhea wrote and overview of the campaign.
He concluded:
"Who was the victor? The answer lies in how one defines winning. One way is to count the numbers. Grant's losses for the campaign were about 55,000 men, and Lee's about 33,000, allowing the rebels to claim a victory of sorts. However, if losses are measured against the respective size of the armies at the outset of the campaign, Lee's losses exceeded 50 percent, while Grant's were about 45 percent. And while each army received substantial reinforcements during the campaign, Grant's capacity to continue to augment his force was vastly greater than Lee's. Despite his many setbacks, Grant lost soldiers at a lower overall rate than Lee, and simple arithmetic indicated that he would prevail."
There's a graphic in the story indicating strength and losses.
At the Wilderness, Grant had 118,000 men, suffered 18,000 casualties; Lee had 67,000 men, suffered 11,000 casualties.
At Spotsylvania, Grant had 100,000 men, suffered 18,000 casualties; Lee had 57,000 men, suffered 12,500 casualties.
At North Anna, Grant had 115,000 men, suffered 2,600 casualties; Lee had 63,900 men, suffered 1,600 casualties.
At Cold Harbor, Grant had 130,000 effectives, suffered 13,000 casualties; Lee had 75,700 men, suffered 5,000 casualties.
This Wikipedia entry might be helpful, too:
http://en.wikipedia.org/wiki/Overland_Campaign
By the way, Rhea's books — 'The Battle of the Wilderness, May 5-6, 1864'; 'The Battle for Spostylvania Court House and the Road to Yellow Tavern, May 7-12, 1864'; 'To the North Anna River, Grant and Lee, May 13-25, 1864'; and 'Cold Harbor, Grant and Lee, May 26-June 3, 1864' are invaluable resources.