These orange booklets can be had covering several different surplus firearm models. They cover the basic care including a detailed tear down of the rifle. As a proud owner of a Lee Enfield SMLE Mk4 Number 2, I own several books regarding this rifle including this one.
This booklet is a quick read with lots of black and white pictures, some pictures are clearer then others. It is a good reference for how to strip down a Lee Enfield Mk1, Mk3 and MK 4. It also has some basic specs on headspace, firing pin protrusion, exploded views and how to make a firing pin disassembly tool. It does not cover anything in the way of serious gunsmithing besides the tear down of the rifle.
The book also discusses, briefly, the origins of the 303 British, and how it bridged the period between the era of Black Powder and the introduction of Nitro based (Smokeless) powders. However, besides the long term storage (burial) of the firearm, it does not really discuss maintenance, or lubrication. Some other things I wish it would have discussed it the proper attachment of a sling, or the use of stripper clips.
Bottom line, do not expect this booklet to be your definitive guide on all things SMLE related, but if you just bought one, and do not know much about your rifle. This is a fairly good introduction to the rifle, the ammunition it fires and a basic tear down.