Eclinpath is probably the best online resource. I think the book Fundamentals of Veterinary Clinical Pathology by Stockham and Scott is the most comprehensive text book. It’s awful to just sit down and read because of the format but it’s great to just look something quick up and has a pretty exhaustive list of differentials.
Maybe make your own flow charts as a study aid? You’ll learn a lot about the information and have a good reference that makes sense to you for future use. Like you could have a heading of Hyperglycemia lead down to categories like endocrine causes (with subcategories of diabetes, hyperadrenocorticism, stress/corticosteroid leukogram and list the mechanism for each), diet (postprandial), other (pancreatitis, sepsis, drugs, etc.). Then repeat for all the main tests. I have some like this another educator gave me to teach rotations from but they’re not something I can share since I didn’t make them.