Basic HL7 Reference Material

I'm not sure what "HL7 Reference Guide" exactly means for you, however, within "health enabled products" (IRIS for Health, Healthhare Connect, Healthshare UCR etc.) there are "HL7 Schemas" for all HL7 v2 variants up to 2.8.2.

Can be accessed from Management Portal Interoperability -> Interoperate -> HL7 v2.x -> HL7 v2.x Schema Structures.

There, for each version you can browse in detail the HL7 message structure, segments, field, subfields etc down to code tables associated to coded entries/fields.

If you don't have IRIS for Health, you can download IRIS for Health community edition that is free for non commercial/production use (check license for details).

As Enrico mentions, HL7 v2 message grammars, segment structures and data type syntaxes are available in IRIS schemas. Complete specifications and semantics are detailed in the normative documents available  on HL7.org web site.