"Witkin" generally refers to a leading California legal treatise by B. E. Witkin titled Summary of California Law, although it may also refer to Witkin's California Criminal Law, California Procedure or California Evidence. All Witkin treatises are published by Thomson/West.
All four Witkin treatises are available on Westlaw. On Westlaw, the database names are WITCRIM, WITEVID, WITPROC, WITSUM -- plus the WITKIN database, which lets you search all four treatises at once. Note: You can pull individual sections using the four specific database names followed by the section number.
The Witkin treatises are also available on Lexis. The database names are CAL;WITCRM, CAL;WITEVD, CAL;WITPRO, CAL;WITSUM and the comprehensive CAL;WITKIN.