The Bentley Manual is probably the best service manual available for the E36, however the cost of them is usually quite steep. Thankfully their is a PDF download available. Also available is the PDF copy of the owners manual. Quite handy as many people no longer have a copy of their owners manual.
E36 Bentley Manual
E36 Owners Manual
E36 Electrical Trouble Shooting Manual
Online TIS Service Manual
If either of the links go dead please let me know. I have a copy of both saved. Please note that the Bentley Manual is a 97MB download.