Perhaps for some things. That range of years is crossing over different models and generations. 1998 and 1999 are essentially the same "generation", but that doesn't preclude the possibility of changes between the years. There might be some carry-over of components and procedures, but there also will be differences.
The Haynes books are good for what they are, but because they cover so much territory in terms of models and years, they are selective in what is actually presented. So you might find some information on a particular job for the earlier model years, but not the later ones. Or there might be "representative" wiring diagrams not one that's identical to your car. As a general source, they can be useful, especially because for what they do include, there's usually photos of real work being done, and the text will reflect this as well, but keep in mind the limitations.
Subaru makes available pdf file versions of its factory service manuals. A complete manual can comprise between 600 - 1000 individual files, (depending on what parts you need/want), and take some time to download file-by-file, but the complete, official, up-to-date factory manual can be obtained for about $35 for a 72-hour download subscription. See Subaru Technical Information System - Welcome
Occasionally someone posts a link to a website where copies (unofficial, and technically contrary to copyright law) can be found. Look here for threads with the words service manual in the title.