The Easy Manual (curently in English and in Japanese) is available in the Download area. This manual is geared at new users and explains the main processes using working examples.

Other Documentation

Ledger123 is an integrated system. In addition to printed sales invoices it will do all related accounting postings and at the end of the year you can get the P&L Statement and the Balance Sheet. While you don't have to use the system for your accounting doing so will save you costs and duplicate data entry. In this scenario you should have a basic understanding of accounting.
Here is an excellent online bookkeeping tutorial: Bean Counter Manual

Armaghan Sabiq has Ledger123 documentation on his site. Check here: SQL-Ledger user and developer docs

Finally you can purchase the original DWS SQL=Ledger manual. It is available here: SQL-Ledger support by DWS