Enron and other big companies have escaped taxes in recent years through financial maneuvers so complex that the Internal Revenue Service has been unable to understand them, the Senate Finance Committee will be told this morning by Congressional tax experts who spent nearly a year going over Enron's tax returns. In a report to the committee, the experts will explain that companies can avoid taxes by exploiting differences in the rules governing the two sets of books that all companies must keep, one for shareholders and the other for the I.R.S., according to people who have seen the report. These...