In addition to a workbook, a good Esperanto dictionary is a must. I'm primarily using two dictionaries in my studies.

The first is John C. WELLS English Esperanto English Dictionary available on Amazon for about thirty dollars. The second is ESPDIC by Paul DENISOWSKI, available here. Though there are print editions, just loading the text file and performing searches for terms of interest has proven extremely useful.

A third dictionary that I think deserves honorable mention is La Bona Lingvo. It's a free online dictionary with alternatives to neologisms.

Lastly, for good practice material I frequently read articles in the Esperanto edition of Wikipedia.

~Jeff

It may be worth mentioning parts 1 and 2 of John Wells' Jen Nia Mondo (with accompanying CDs).